Preheat oven to 375 F.
Pour 2 tablespoon of olive oil in a 9" cast iron skillet. Saute diced sausage over medium heat until browned, stirring often. Transfer the browned sausage to a bowl and set aside.
Heat the remaining 1 tablespoon of olive oil to the pan. Cook the onion and sweet potato on medium-low heat for 15 minutes, or until the potato is soft, stirring often.
Add the sausage back to the pan and add the spinach leaves. Mix to wilt the spinach.
Beat the eggs in a bowl and season with salt and pepper. Add them to the pan with the sausage and vegetables.
Cook the frittata for 5-7 minutes, until the edges are set.
Transfer the pan to the oven and bake for 20-25 minutes, until the frittata is fluffy and golden.
