Preheat your oven to 375 degrees F. Grease a 9x13 inch baking dish with cooking spray or a thin coat of butter and set it aside.

In a large skillet over medium-high heat, cook the bulk breakfast sausage, breaking it apart with a wooden spoon as it cooks. Cook for 6 to 8 minutes until no pink remains and the sausage has some browning on the edges. Drain off any excess grease and set the sausage aside.

Scatter the cooked sausage evenly across the bottom of the prepared baking dish. Layer the diced red bell pepper, green bell pepper, and onion over the sausage. Sprinkle 1 cup of the shredded cheddar on top of the vegetables.

In a large mixing bowl, crack all 10 eggs and add the milk, garlic powder, onion powder, salt, pepper, and red pepper flakes if using. Whisk until the mixture is fully combined and slightly foamy, about 30 seconds.

Pour the egg mixture slowly and evenly over the layered ingredients in the baking dish. Use a fork to gently press any vegetables or sausage that float back down under the egg. Sprinkle the remaining 1/2 cup of cheddar over the top.

If baking right away, place the dish on the center rack and bake for 35 to 40 minutes. The casserole is done when the center is set and does not jiggle when you gently shake the pan. The top should be golden and slightly puffed. If you press the center lightly with your finger, it should feel firm.

Remove from the oven and let it rest for 5 minutes before slicing. This resting time is important: the egg continues to set as it cools and you will get clean, even slices instead of a runny mess. Cut into squares and serve warm.