Set your oven to 400°F. Lightly grease a 9x9 inch baking dish. Place the diced sweet potatoes on a baking sheet, drizzle with olive oil, and sprinkle with 1/4 teaspoon salt and pepper. Toss to coat. Roast for 20 to 25 minutes, until fork-tender and edges are lightly caramelized.

While the sweet potatoes roast, heat a skillet over medium heat. Add a splash of oil and sauté the onion and bell pepper for 4 to 5 minutes, until softened. Add the spinach and cook just until wilted, about 1 minute. Remove from heat. If using meat, brown it in the same skillet, then drain excess grease.

In a large bowl, crack the eggs. Add the milk, remaining 1/4 teaspoon salt, garlic powder, and smoked paprika. Whisk vigorously until the mixture is frothy and uniform.

Spread the roasted sweet potatoes evenly in the prepared dish. Scatter the sautéed vegetables (and meat if using) over the top. Pour the egg custard over everything, gently shaking the dish to distribute. Sprinkle the shredded cheddar evenly over the surface. If using cream cheese, drop small dollops over the casserole and swirl with a knife.

Place the dish in the oven and bake at 375°F for 30 to 35 minutes, until the center is set and the edges are puffed and browned. A knife inserted in the center should come out clean.

Let the casserole rest for 5 to 10 minutes. This allows the eggs to set completely, making slicing easier. Cut into squares and serve warm.