In a large, non stick skillet, melt butter; saute mushrooms, onion and garlic over medium heat until golden. Stir in flour and cook for 1 minute. Gradually stir in milk and cook until mixture comes to a boil and thickens, stirring constantly. Stir in mustard and season with salt and pepper. Add spinach. With back of a spoon, make 8 indentations in spinach mixture. Break one egg into each well. Cover and cook on medium low for 6 to 8 minutes or until egg whites are set.