Preheat oven to 350 degrees Fahrenheit (175 degrees Celsius) and spray a 9×13 inch baking dish with cooking spray.
In a large skillet, brown breakfast sausage until cooked through. Drain any excess grease.
Add diced onion and diced bell pepper to the sausage and cook for 3-5 minutes until softened.
In a large bowl, combine the cooked sausage mixture, thawed hash browns, and shredded cheddar cheese.
In a separate bowl, whisk together eggs, milk, salt, and pepper until well combined.
Pour the egg mixture over the sausage and hash brown mixture, ensuring everything is thoroughly coated.
Transfer the mixture to the prepared baking dish and spread evenly.
Bake for 45-55 minutes, or until the casserole is set in the center and golden brown on top.
Let the casserole cool for a few minutes before slicing and serving to allow it to set completely.