Mushroom Casserole

1½ lbs. mushrooms, cleaned
½ sm. Onion, chopped
½ stick butter
8 oz. Shredded mozzarella
8 oz. Shredded cheddar
3 slices cubed bread
½ cup sour cream

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Sauté mushrooms stems and chopped onion in butter.  Add bread cubes and sour cream and heat through.  Add all mushrooms and mix well with salt, pepper and dash of garlic powder.  Turn into a 13 x 9 Pyrex.  Top with shredded cheeses.  Bake at 375° for 15-20 minutes until bubbly and lightly browned.