Preheat oven to 375 degrees F
Place half of the butter, mushrooms and onion in a pan
Sauté until tender
Remove from heat, add the cream cheese and mix thoroughly
Mix in the other three cheeses, allow to melt (turn the heat on low if necessary), then mix in the spinach and garlic
Spread the prosciutto out on a cutting board, layer the flank steak over it
Layer the mushroom and cheese mixture over the flank steak
Roll the meats to make a flattened tube with the mixture on the inside
Place the other half of the butter in a cooking pan, melt it
Cook the meat on high the cheese has melted and the meat is no longer red (about 20 minutes)