Heat butter or oil in large saute pan at medium high heat. Add squash and broccoli to pan and continue to cook until tender, approximately 8 - 12 minutes. While squash is cooking, lightly beat egg in a small bowl. Add cheese, milk, salt and dry mustard to the egg and stir until thoroughly combined. Pour mixture over zucchini and broccoli and continue to heat until cheese is melted and bubbly. Add salt and pepper to taste.
Note: The cheese and milk mixture can be replaced using a tomato or pasta sauce with basil and garlic for seasoning.