Under broiler or in toaster oven, toast muffins for about 1 minute or until golden. Spread each with 1 tbsp (15 mL) tomato sauce; set aside.
In non stick skillet, melt butter over medium heat; cook mushrooms and green pepper, stirring occasionally, for about 5 minutes or until softened.
In bowl, whisk together eggs, milk, salt and pepper until combined; add to pan and cook, stirring with spatula to form large soft curds, for about 1 minute or until eggs are thickened and moist but no visible liquid remains.
Spoon eggs over muffins. Sprinkle with mozzarella cheese; broil for about 1 minute or until cheese is melted.