In a large skillet over medium-high heat, cook the sausage, breaking it up with a spoon, until browned and cooked through, about 8-10 minutes. Add the diced onion and bell pepper during the last 3-4 minutes of cooking to soften. Drain any excess grease and let cool slightly.

Grease a 9x13 inch baking dish. Spread the thawed hash browns evenly in the bottom. Sprinkle with 1 1/2 cups of the shredded cheddar cheese. Evenly distribute the cooked sausage and vegetable mixture over the cheese layer.

In a large bowl, whisk together the eggs, milk, salt, pepper, garlic powder, and paprika until completely smooth and well combined.

Slowly pour the egg mixture evenly over the sausage and hash brown layers in the baking dish. Top with the remaining 1/2 cup of cheddar cheese and all of the mozzarella cheese. Cover tightly with plastic wrap or aluminum foil and refrigerate for at least 8 hours, or overnight.

Preheat oven to 350°F (175°C). Remove the casserole from the refrigerator and let it sit at room temperature for 20-30 minutes while the oven heats. Bake, uncovered, for 50-60 minutes, or until the center is fully set, the edges are golden brown, and a knife inserted in the center comes out clean.

Let the casserole rest for 10 minutes before slicing. Garnish with chopped fresh chives. Serve warm with hot sauce on the side if desired.