Preheat oven to 350 F.
Cook mushrooms, zucchini, and onions in a small pan with olive oil and a sprinkle of salt and pepper for 3-4 minutes or until they soften slightly. Add to the bottom of a pie pan or glass baking dish.
In a bowl, combine eggs and milk and whisk together until a smooth mixture. Pour on top of vegetables. Sprinkle cheese on top and lay the sliced tomato on top of that. sprinkle top with salt and pepper.
Bake 40 minutes or until the egg is cooked in the middle.