I took the 'I'm allergic to that' route. Some people are food pushers and that is the best way for me to keep them from 'forcing' stuff on me I don't want.

Maybe your wife could intervene and re-inforce to her mother that she should cook everything as usual and that you will make your own choices.

Some people act as if we will starve to death if we don't eat a complete meal. Not like I can't go home in a few hours and get what I want!