1 medium onion chopped
3 cups mushrooms chopped
3 tablespoons plus more for buttering the bread
¼ cup dried parsley or you can used ½ cup fresh parsley, chopped fine
2 egg yolks
¾ cup shredded mozzarella cheese
¼ cup shredded parmesan cheese
1 teaspoon salt
6-8 slices sandwich bread crust cut off and flattened slightly with a rolling pin