Saute onions and garlic in about 2 tbs. of olive oil or butter till tender. Add sliced mushrooms and cook about 5 min. more. I like to cover my skillet with a lid to hurry along the process.
Slice bread in half, slightly brush with olive oil or butter. Sprinkle grated mozzarella, add onions, garlic and mushrooms. Top with a little more mozzarella and some Parmesan and red pepper flakes. Place under broiler just long enough to melt cheese.