Preheat oven to 375°F and grease a 9×9-inch baking dish.
Heat oil in a pan 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 if needed.
Add onion and bell pepper to sweet potatoes and sauté for 3–4 minutes.
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 evenly.
Bake the Sweet Potato Breakfast Casserole for 20–24 minutes until set.
Let rest for 5 minutes, slice, and serve with avocado if desired.