Creating Sheet Pan Frittata is straightforward if you follow these simple steps:

1. Preheat Oven: Preheat your oven to 375°F (190°C) and grease a large rimmed baking sheet with cooking spray or olive oil.
2. Whisk Eggs: In a large bowl, whisk together the eggs, milk, salt, pepper, and garlic powder until well combined.
3. Prepare Vegetables: Chop the bell peppers, zucchini, spinach, and onions into small, uniform pieces for even cooking.
4. Combine Ingredients: If using, add the cooked sausage or bacon to the egg mixture. Then, fold in the diced vegetables and half of the cheese.
5. Pour Mixture: Pour the egg and vegetable mixture into the prepared baking sheet, spreading it out evenly.
6. Sprinkle Cheese: Top with the remaining shredded cheese, distributing it evenly over the surface.
7. Bake: Place the baking sheet in the preheated oven and bake for 25-30 minutes, or until the frittata is set and lightly golden on top.
8. Cool Slightly: Once done, remove the pan from the oven and let it cool for a few minutes.
9. Slice: Cut the frittata into squares or wedges for serving.
10. Garnish: Sprinkle fresh herbs on top for a pop of color and added flavor.

These steps will guide you in creating this incredible frittata effortlessly.