Preheat and prep: Heat your oven to 400°F (200°C). Lightly grease a 9x13-inch baking dish with cooking spray or oil.

Roast the sweet potatoes: Toss diced sweet potatoes with 1 tablespoon olive oil, smoked paprika, salt, and pepper. Spread on a sheet pan and roast for 18–22 minutes, until just tender and lightly browned. Reduce oven to 375°F (190°C) after roasting.

Cook the sausage: In a large skillet over medium heat, add the sausage. Cook, breaking it up with a spatula, until browned and cooked through, 6–8 minutes. Drain excess fat if needed.

Sauté aromatics: In the same skillet, add 1 tablespoon olive oil if the pan is dry. Stir in onion and bell pepper. Cook 4–5 minutes until softened. Add garlic powder and thyme; cook 30 seconds more. Stir in spinach just until wilted, about 1 minute. Season with a pinch of salt and pepper.

Whisk the eggs: In a large bowl, whisk eggs, milk, 1/2 teaspoon salt, and 1/4 teaspoon black pepper until smooth. Stir in half of the cheese, if using.

Assemble the casserole: Spread roasted sweet potatoes evenly in the prepared baking dish. Top with cooked sausage and the sautéed veggies. Pour the egg mixture over the top. Sprinkle with the remaining cheese.

Bake: Place in the 375°F (190°C) oven and bake 28–35 minutes, until the center is set and the edges are lightly golden. A knife inserted in the center should come out clean.

Rest and serve: Let the casserole rest 10 minutes before slicing. Garnish with green onions or parsley and serve warm.