Step 1: Preheat your oven to 350°F (175°C). Grease a 9x13 inch baking dish with cooking spray or butter.
Step 2: In a large skillet over medium heat, cook the breakfast sausage until browned and fully cooked, breaking it apart with a spoon as it cooks. Drain excess grease and set aside.
Step 3: In a large mixing bowl, whisk together the eggs, milk, salt, black pepper, garlic powder, and onion powder until well combined.
Step 4: Stir in the cooked sausage, hash browns, diced bell peppers, diced onions, and half of the shredded cheddar cheese into the egg mixture. Mix until everything is evenly coated.
Step 5: Pour the mixture into the prepared baking dish, spreading it out evenly. Top with the remaining shredded cheddar cheese.
Step 6: Bake in the preheated oven for 45-55 minutes, or until the casserole is set in the center and the top is golden brown. A knife inserted into the center should come out clean.
Step 7: Allow the casserole to cool for about 10 minutes before slicing. Garnish with chopped green onions before serving.