Scramble eggs and season with salt and pepper. Set aside in oven-safe casserole dish.
Cook onion in a skillet with olive oil. Add mushrooms, zucchini, and tomatoes. Cook for 15 minutes, then cool for 10 minutes.
Add vegetable mixture to eggs. Add mozzarella and scallions.
Cook at 350℉ for 25 minutes.
Top with parsley and sliced avocado, and serve.