A woman was punched in the face by an enraged McDonald’s cashier when she demanded cheese on her young son’s Happy Meal cheeseburger, her lawyer said Tuesday in opening arguments of her civil case against the Brooklyn franchise.

Gui Ying Shi, 35, sued the Sunset Park McDonald’s after cashier Marisol Acosta hit her for complaining that her young son’s Happy Meal contained a hamburger instead of a cheeseburger, lawyer Lawrence Glynn said in the Brooklyn Supreme Court trial.

“She starts cursing at her, calls her stupid, said, ‘You need to learn to speak English,’ ” Glynn said. “Ms. Acosta throws a left cross to Ms. Shi’s right jaw. Knockout! Knocks out a tooth.”

The McDonald’s franchise is liable for Acosta’s actions, Glynn said, because none of the three managers present tried to stop the violence.

But defense attorney Michael Stonberg argued that the “punch” was more of a lunge or a slap — and that the franchise responded by immediately firing Acosta.

“It goes without saying that Marisol Acosta was not authorized to get into an altercation with a customer,” he said.

Police responded to the McDonald’s and Acosta was charged with assault, cops said.