Spray 9 x 13 baking dish with cooking spray and preheat oven to 350 degrees F. Add sausage, potatoes and bell peppers into the dish spreading evenly.
In a large mixing bowl, whisk the eggs. Then add milk, garlic powder, salt, pepper and whisk until well combined.
Pour egg mixture over veggies. Sprinkle with red onion and cheese. Bake for 40 minutes.
Remove from the oven, cut into 8 slices and enjoy warm or cold.
Notes
Store: Refrigerate leftovers in an airtight container for up to 5 days.
Freeze: Leftovers for up to 3 months. Thaw on a counter for 4-5 hours or in the fridge overnight. Reheat in a microwave, toaster oven or bake in regular oven at 350 degrees for 15 minutes.
Make ahead: Refrigerate casserole assembled and covered with plastic wrap for up to 2 days. Bring to room temperature on a counter for 30 minutes and bake as per recipe directly from the fridge. It may require extra 10-15 minutes cook time.