Earlier this year, de Blasio tweeted about his support for "Meatless Mondays" in Big Apple schools -- where students would be given a "meatless" lunch one day per week.
"You know how I know #MeatlessMondays is a good idea? Because our students love it and Sean Hannity hates it," de Blasio wrote in a Twitter message. "Meatless Mondays: coming to all 1,800 NYC schools this fall."
Wednesday on "Hannity," the Fox News host asked de Blasio about the tweet, challenging the mayor to a bet.
Hannity projected that if a poll of all New York City schoolchildren was taken, they would support the host's weekly meal plan over the mayor's.
"You willing to put money on that?" Hannity asked.
Hannity jokingly offered several alternatives to "Meatless Mondays," like "McDonald's Mondays," and "Wendy's Wednesdays."
The mayor smiled, quipping, "You really are a threat to society."
"By doing this, we are helping kids to be healthier," de Blasio added, defending his plan over Hannity's. "We have an obesity problem in this country."
Regarding the initiative, City Hall said it will provide the school system’s one million students with all-vegetarian breakfast and lunch menus every Monday.
Advocates note Meatless Mondays promotes healthy, environmentally friendly meal options.
“The announcement that Mayor de Blasio has expanded Meatless Monday to all New York City public schools marks a major milestone," said Sid Lerner, founder of the Meatless Monday Movement, in a statement. "Through this united effort, New York City’s schools take a leadership role in getting our children on a healthier track, as well as making a positive impact on our environment."
Fox News' Frank Miles contributed to this report.