Preheat oven 350 degrees
Dice onion, mushrooms, and garlic
Cook down onions in butter for 5 minutes. Add minced garlic and mushrooms. Cook on low heat for 20 minutes.
Add cream cheese, Swiss, mozzarella, and parmesan. Cook until melted. Add spinach and cook until wilted.
On your cutting board salt and pepper flank steak. Lay out flat. Lay prosciutto across top park of steak.
Pour cheesy filling across steak and prosciutto. Spread evenly across top.
Using skewers to poke through folding up both sides keeping the steak together and filling in the center.
Bake for 25-30 minutes