The Menu Cheeseburger Explained
Margot realizes that Slowik was the happiest when he served regular people good old American cheeseburgers. Being requested for one, Slowik enjoys the nostalgically pleasurable cook. Margot also smartly asks to pack the rest to go, telling Slowik she doesn't want to waste good food and so he spares her.
Over time, Slowik served the snobbish elite who didn't really care about his art. Margot was originally not part of the guest list. She's an escort Tyler invited at the last moment in place of his original date.
Margot is not a pretentious diner like the rest, and this makes Slowik very restless as she's not falling in place with his brutal plan for the evening. She also sees a picture of Slowik as a young happy cook making burgers. After years, Chef Slowik experiences absolute joy in cooking and serving food, and that's why he allows Margot to leave.
Here's the detailed explanation - The Menu explained.