Combine tuna, mushrooms and cheeses in a greased 10-inch quiche OR 8-inch square baking dish.
Combine milk, flour, eggs, onion, parsley, garlic salt and cayenne pepper in blender and process 15 seconds OR beat 1 minute with an electric mixer. Pour over tuna mixture.
Bake 35 to 40 minutes if using quiche pan, (60 minutes if using 8-inch square pan), or until knife inserted in center comes out clean. Let set 5 minutes before serving. Garnish with fresh tomato slices, if desired.