Brown ground sausage over medium heat in a cast-iron skillet. Remove meat from the pan and drain the fat. Set aside. Saute the onion, peppers, and zucchini. Add spinach and let it cook into the mixture for 1-2 minutes.
While vegetables are cooking, steam-cook broccoli in the microwave for 1-2 minutes. Drain when it is done. Now, mix the cooked broccoli and sausage into the veggie mixture and spread it evenly in the skillet.
Whisk together the eggs, egg whites, milk, garlic powder, salt, and pepper. Add cheese to the top of the sausage and veggies. Pour the egg mixture over all the ingredients. Top with parmesan cheese if using. Cook for 3-5 minutes on the stovetop.
Place the skillet in the oven and bake at 400°F for 15-20 minutes. Once your frittata is cooked through, turn the oven to broil, and cook an additional 1-2 minutes until golden brown. Let cool slightly and enjoy!