Preheat the oven to 375°F (190°C). Lightly grease a 9×9-inch baking dish with oil or non-stick cooking spray.
Heat the oil in a medium skillet over medium-high heat.
Add the diced sweet potatoes and a small pinch of salt. Cover and cook for 10–12 minutes, stirring occasionally, until slightly tender.
In a separate skillet, cook the ground lamb with garlic powder, Italian seasoning, fennel, sage, salt, black pepper, and cayenne until browned and fully cooked.
Add the red onion and bell pepper to the sweet potatoes. Cook for 3–4 minutes until softened.
Stir in the spinach and cook for 1–2 minutes until wilted.
Add the cooked lamb mixture to the vegetable mixture and stir well. Remove from heat.
In a large bowl, whisk the eggs with a pinch of salt and pepper.
Transfer the sweet potato mixture to the prepared baking dish.
Pour the whisked eggs evenly over the top, ensuring all ingredients are submerged.
Bake the Sweet Potato Breakfast Casserole for 20–24 minutes, or until the center is fully set.
Remove from the oven and allow it to rest for 5 minutes before slicing.
Serve warm with sliced avocado and your favorite hot sauce if desired.