In a skillet, melt the butter. Saute the diced onions for about 7 minutes or until lightly browned, seasoning with salt & pepper while cooking. Stir in the minced garlic & cook for an additional 1-2 minutes. Add in the chopped mushrooms, season with a little more salt & pepper. Cook for 7-9 minutes, until the mushrooms have lightly browned and are tender.