Preheat your oven to 400°F. Lightly grease a 9×13-inch baking dish to prevent sticking.

Evenly spread the frozen shredded hash browns in the prepared dish. There’s no need to thaw them beforehand. In a small microwave-safe bowl, melt the butter, then drizzle it over the potatoes. Use a spatula or your hands to toss and coat the potatoes well. Season with salt and pepper to taste.

Transfer the dish to the oven and bake for 30-35 minutes, or until the potatoes are mostly cooked and the edges begin to turn golden brown. Once done, remove from the oven and reduce the temperature to 350°F.

While the potatoes are baking, chop the onion, green bell pepper, and red bell pepper into small, uniform pieces.

In a large skillet over medium-high heat, add the breakfast sausage along with the chopped vegetables. Break the sausage into crumbles as it cooks. Sauté for about 5-7 minutes, until the sausage is browned and cooked through and the onions have softened. Drain any excess grease from the pan.

Spread the sausage mixture evenly over the cooked hash browns. You can leave it as a distinct layer or mix it into the potatoes for an even distribution.

Generously sprinkle the Monterey Jack and sharp cheddar cheese over the sausage layer, ensuring even coverage.

In a large mixing bowl, whisk together the eggs, half-and-half (or preferred dairy option), seasoned salt, kosher salt, black pepper, and dry mustard (if using). For a smoother mixture, you can blend it in a stand mixer or blender.

Slowly pour the egg mixture over the casserole, making sure it spreads evenly across the surface and soaks through the layers.

Place the dish in the preheated 350°F oven and bake for 35-45 minutes. The casserole is done when the edges turn golden and bubbly, and the center is set with minimal jiggle when gently shaken.

Once baked, remove from the oven and let it rest for 5 minutes before serving.