My MIL is currently giving me the silent treatment because I won't share a "family recipe" that is actually just expensive takeout

I'll die if I don't share this... Three years ago I was supposed to cook for my wife’s birthday and I completely torched the main dish. In a total panic, I drove to this Italian deli two towns over, bought two massive trays of lasagna, scraped the cheese on top so it looked messy/homemade, and hid the foil containers in the outside trash. It was a hit. Like, they wouldn't shut up about it. Best thing they ever ate. Now I am the Lasagna Guy. Every birthday, every holiday. I have to drive 40 minutes, smuggle this stuff into the house in a gym bag, and transfer it to my own Pyrex dishes while my wife is in the shower. The issue is my MIL asked for the recipe last week for her church potluck. I didn't know what to do so I told her I promised my late grandmother on her deathbed that I wouldn't write it down. She called me selfish and is actually pissed off at me. I feel like a total fraud but I can’t tell her the "secret ingredient" is $80 and a guy named Sal.