Step 1: Preheat your oven to 375°F (190°C). Lightly grease a 9×13 inch baking dish.
Step 2: In a large skillet, cook the breakfast sausage over medium-high heat, breaking it apart with a spoon as it browns, until no pink remains. Drain any excess grease thoroughly.
Step 3: Add the chopped onion and bell pepper to the skillet with the cooked sausage. Cook until the vegetables are softened, about 5-7 minutes. Season with salt and black pepper to taste.
Step 4: In a large bowl, whisk together the eggs, milk, Dijon mustard, garlic powder, onion powder, and a pinch of salt and pepper. Whisk until well combined.
Step 5: Spread the cooked sausage and vegetable mixture evenly in the prepared baking dish. Sprinkle 1.5 cups of the shredded cheddar and all of the Monterey Jack cheese over the sausage mixture.
Step 6: Carefully pour the whisked egg mixture over the cheese and sausage layer, ensuring it distributes evenly.
Step 7: Sprinkle the remaining 0.5 cup shredded cheddar cheese over the egg mixture. Arrange the frozen tater tots in a single layer over the cheese.
Step 8: Bake for 40-50 minutes, or until the tater tots are golden brown and crispy, and the egg mixture is set. A knife inserted into the center should come out clean. If the tots are browning too quickly, you can loosely cover the casserole with foil for the last 10-15 minutes.
Step 9: Remove the casserole from the oven and let it rest for 5-10 minutes before serving. Garnish with fresh chopped parsley or green onions, if desired. Slice and enjoy your ultimate cheesy sausage tater tot casserole!