“Iron Chef” Masaharu Morimoto had an interesting challenge at Cipriani Downtown on Wednesday when he butchered a 70-pound tuna under kosher restrictions. He was invited by the Lubavitch Youth Organization to make sushi, sashimi, and carpaccio out of the fish, which was assessed and approved by chief Rabbi of Israel Yonah Metzger. “I spent weeks studying kosher principles to make sure I got it right,” Morimoto told Page Six. Mayor Bloomberg and hotelier Morris Moinian were also at the dinner.