Preheat oven to 325°F.
Spray a 9×9-inch casserole dish with a good coating of cooking spray, and set aside.
In a medium skillet, add the olive oil over medium heat. Add the onions, and cook until they are soft.
Then, add the rest of the veggies, and sauté until they are tender. Add this veggie mixture to the baking dish. Set aside.
While the veggies are cooking, whisk together the eggs, almond milk, garlic powder, mustard powder, salt, and pepper in a bowl.
Pour the egg mixture over the veggie mixture in the baking dish.
Next, add the cheese.
Cover with foil, and bake 40 minutes. Remove the foil, and cook an additional 10 minutes, or until the egg mixture is cooked through.
Let it cool for 5 minutes before serving.