Preheat the oven to 375°F (190°C) and grease a 9x13 inch baking dish with butter or non-stick spray.
In a skillet over medium heat, sauté the diced onion and bell pepper until softened, approximately 5 minutes; add spinach and cook until just wilted.
In a large mixing bowl, whisk together the eggs, milk, sour cream, garlic powder, salt, black pepper, and smoked paprika until well combined.
Place the thawed hash browns in an even layer at the bottom of the prepared baking dish.
Distribute the sautéed vegetables evenly over the hash browns.
Sprinkle 1.5 cups of the shredded cheddar cheese over the vegetable layer.
Pour the egg mixture slowly over the hash browns and vegetables to ensure even coverage.
Top with the remaining 0.5 cup of cheddar cheese.
Bake for 40 to 45 minutes, or until the center is firm and the cheese is golden and bubbling.
Remove from the oven and allow the casserole to rest for 10 minutes before slicing.