Preheat the oven to 375°F. Grease a 9x13-inch baking dish with cooking spray or a light coating of oil, then set it aside.
Heat a large skillet over medium-high heat. Cook the sausage until browned and no longer pink, breaking it into crumbles as it cooks. Drain excess grease with a paper towel and transfer the sausage to the prepared baking dish.
In the same skillet, add one tablespoon of oil. Add the sweet potatoes, season lightly with salt, and cook for about 12 minutes, stirring occasionally, until tender. Transfer to the baking dish.
In the same skillet, add the spinach (add 1–2 tablespoons of water if needed) and cook for 1-2 minutes, until wilted. Transfer to the baking dish.
In a medium bowl, whisk together the eggs, milk, about ½ teaspoon salt, and ¼ teaspoon pepper. Stir in the cheese.
Pour the egg mixture over the sausage, sweet potatoes, and spinach. Stir gently so all ingredients are submerged.
Bake for 20-30 minutes, or until the eggs are set and the top is lightly golden. Let stand for 5–10 minutes before serving.