Preheat oven to 375°F (190°C) and grease a 9×9-inch baking dish.
Heat oil in a skillet over medium-high heat. Add sweet potatoes, sprinkle with salt, cover, and cook for 10–12 minutes until slightly tender.
In another pan, cook the ground lamb or beef sausage with seasonings until browned. Drain excess fat.
Add onion and bell pepper to the sweet potatoes and cook for 3–4 minutes until softened.
Stir in spinach and cook until wilted. Add cooked sausage and remove from heat.
In a bowl, whisk eggs with salt and pepper.
Transfer the mixture into the baking dish and pour eggs over the top, ensuring everything is evenly covered.
Bake the sweet potato breakfast casserole for 20–24 minutes until the center is set.
Let it rest for 5 minutes before slicing. Serve with avocado if desired.