Preheat and prep: Heat your oven to 375°F (190°C). Lightly grease a 9x13-inch baking dish with olive oil or line it with parchment.
Roast the sweet potatoes: Toss the cubed sweet potato with 1 tablespoon oil and 1/2 teaspoon salt. Spread on a sheet pan and roast for 15–18 minutes, until just tender and lightly browned at the edges.
Set aside.
Brown the sausage: Heat a large skillet over medium heat. Add the sausage and cook, breaking it up with a spatula, until browned and cooked through, about 6–8 minutes. If using plain ground turkey/chicken, add 1/2 teaspoon salt, 1/2 teaspoon paprika, and 1/2 teaspoon cumin while cooking.
Remove sausage with a slotted spoon; leave a little fat in the pan.
Sauté the vegetables: In the same skillet, add remaining 1 tablespoon oil if needed. Add onion and bell pepper with a pinch of salt. Cook 4–5 minutes until softened.
Stir in garlic and cook 30 seconds, then fold in spinach just until wilted. Remove from heat.
Mix the egg base: In a large bowl, whisk eggs, coconut milk, smoked paprika, oregano, cumin, remaining salt, black pepper, and red pepper flakes if using.
Assemble the casserole: Spread roasted sweet potatoes evenly in the baking dish. Layer on the sausage, then the sautéed vegetables.
Pour the egg mixture over the top, tilting the dish to distribute evenly. Gently tap the pan to remove air pockets.
Bake: Place on the center rack and bake 28–34 minutes, until the eggs are set and the center no longer jiggles. A knife inserted in the center should come out clean.
Rest and garnish: Let the casserole rest 10 minutes before slicing.
Top with chopped parsley or chives if you like.
Serve: Cut into squares and serve warm. Add avocado slices or a spoonful of salsa for extra richness and freshness.