The National Labor Relations Board has issued a complaint against the New York City restaurant group headed by the acclaimed chef Ignacio Mattos, saying that it engaged in illegal practices to dissuade workers from forming a union at Lodi, in Rockefeller Center.

The 24 allegations, some involving Mr. Mattos himself, include surveilling workers’ communications, telling employees that the restaurant would close if a union were formed and warning undocumented workers that their immigration status would be affected if they unionized.

The practices alleged are fairly common anti-union tactics. What is distinct about the case, though, is the new tool at the N.L.R.B.’s disposal: its recent ruling that lowers the bar for a union to win recognition.

The ruling, known as the Cemex decision for the construction-materials corporation it was first used against last August, allows the N.L.R.B. to order a company to recognize and bargain with a union — even when workers have voted a union down, as they narrowly did last year at Lodi — if the board’s general counsel can prove to an administrative law judge that management used unlawful union-busting methods that affected the election’s outcome.

In April, the regional director of the board’s Manhattan office, John Doyle, issued the complaint, which seeks a Cemex order. Administrative law judges have imposed Cemex orders on three companies, but this is its first case involving a restaurant. If a judge deems that Mattos Hospitality acted unlawfully, the company will have to bargain with workers at Lodi — which would become one of only a few independent restaurants in New York with a union.

The bargaining unit would be small, at least at first — roughly 50 workers at Lodi, out of 228 employees at Mattos Hospitality’s three restaurants. And a significant percentage of unions never win a contract, said Jeffrey Hirsch, a professor at the University of North Carolina who specializes in labor and employment law.

But the precedent of a Cemex order against a restaurant, he said, could galvanize other food-service workers to unionize. “With Cemex, you go straight to bargaining, and that is a big change,” Mr. Hirsch said.

The N.L.R.B. said that several fledgling unions have been seeking Cemex decisions since the August ruling, and that its general counsel is currently seeking Cemex orders in about a dozen cases nationally.

Mattos Hospitality said it couldn’t comment on the allegations while the case is active. But it pointed to a statement it issued before the union vote that said, “We do our best to make sure that every team member is treated with dignity and respect, and every employee has a voice.”

“The choice of whether the team wishes to be represented by a union has always been up to them, and we are committed to preserving their right to make an informed decision. The Lodi team voted not to unionize in a free and fair election administered by the N.L.R.B.” (The vote was 25 to 21 against the union.)

An innovative chef from Uruguay, Mr. Mattos first made his mark in 2013 by opening Estela, where guests included the Obamas. He has been regularly profiled in the media, and in April was named one of the 100 most culturally influential people in the country by Cultured Magazine.

Lodi, an Italian cafe, was one of the first restaurants to open as part of the recent high-profile overhaul of Rockefeller Center Plaza. (The federal case against Lodi does not include Estela or Mr. Mattos’s third restaurant, Altro Paradiso.)

Localized union-organizing drives have been much in the news, as workers at outposts of major companies like Starbucks and Amazon have won long-fought battles. Yet just 3.6 percent of food-service workers in the United States belong to unions, according to a 2023 report from the U.S. Bureau of Labor Statistics, compared with 10 percent of all workers. This is in part because the restaurant business has high turnover, and the numbers of workers at independent restaurants are often too small for larger trade unions to take on.

The Lodi workers are affiliating with Restaurant Workers Union — Sindicato de Trabajadores de Restaurantes, Local 1, a small group founded in 2020, rather than working with a larger union. This can be a good fit at a small restaurant, said Tareq Saghie, a New York City organizer for Restaurant Opportunities Center United, an advocacy and education group for hospitality workers, as it puts workers in more direct communication with management and allows the creation of a union that’s tailored to the type of restaurant.

But these workers don’t have the experience or resources of a larger union like Unite Here Local 100, which represents several well-established restaurants in New York City, including the Grand Central Oyster Bar and Shun Lee Palace, he said.The Lodi workers said they are helping others in at least four New York restaurants to form unions.

Typically, when workers notify an employer that they are organizing a union, management can either recognize the union or ask for an election.

At Lodi, the atmosphere was relatively amiable before workers notified management about their effort in January 2023, said Rose Thomas, a baker there from June 2022 to June 2023. “We didn’t have any managers that were yelling or verbally abusive or anything along those lines.” Employees said they were seeking better and more equitable pay, benefits like health insurance and safer working conditions.

But once management asked for a union election, the environment quickly turned hostile, Ms. Thomas said. According to the complaint issued by the N.L.R.B. — which is backed by dozens of recordings, screen shots and sworn affidavits from workers — managers began supervising workers more closely, referencing private text messages between workers and making employees feel they were under surveillance.

The complaint states that management “appealed to racial prejudice” to discourage employees from joining the union. In affidavits submitted to the board, workers said the restaurant hired an anti-union consultant, who identified himself by a false name. The consultant held meetings with Latino employees — recordings of which were heard by The New York Times — in which he cautioned them not to trust English-speaking co-workers who had joined the union effort.

In interviews, Latino workers said Mr. Mattos invoked his identity as a fellow Spanish-speaking immigrant. Sometime after the union announcement, one worker said in a sworn statement to the board, Mr. Mattos stormed up to him, pointed a finger and said, “Eso no se hace” (“We don’t do this.”).

Eric Schmidt, a server at Lodi, has taken a lead role in organizing the union, and has a second job as a catering waiter. In a February 2023 text message viewed by The Times, Mr. Mattos told that other employer that Mr. Schmidt had betrayed him by aiding the union effort. The employer, who asked not to be named, said in an interview that she felt Mr. Mattos was warning her against working with Mr. Schmidt. (She kept Mr. Schmidt on anyway.)

The union says three-quarters of the workers at Lodi signed cards saying they wanted the group to represent them. But employees voted against the union in February 2023, and that month, workers began filing charges of unfair labor practices with the National Labor Relations Board.

An administrative law judge plans to hear the case beginning June 24, and it could take months for the judge to make a decision. If the judge issues a Cemex order, Mattos Hospitality could appeal the decision to the full labor board and delay bargaining, said Mr. Hirsch, the law professor.

A Cemex order “lowers the barriers” to a successful union, he said. “It doesn’t lower all of them.”

Christina Morales contributed reporting.

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