Preheat oven to 425 degrees and line a baking sheet with foil. Place mushrooms stem side down and cook for 10-12 minutes or until soft
While mushrooms are cooking, melt butter in a large skillet; add onions and cook for 5 minutes. Add peppers and cook for 2 minutes more
Beat eggs with garlic salt and add to skillet; cook until eggs are almost set. Stir in spinach and cheese and cook for 1 minute more or until cheese is melted and spinach is lightly wilted. Turn mushrooms over and fill with eggs.