Preheat oven to 400 degrees F.
In a bowl, whisk the eggs with red pepper flakes and salt & pepper.
Melt butter in a cast iron skillet (or any oven proof skillet) over medium high heat. I used a 10-inch skillet.
Add onion, garlic, and sweet potatoes and cook until they are soft, about 5 minutes.
Add the sausage, stirring frequently until the slices are evenly browned, about 8 minutes.
Add the spinach and stir until it's wilted and cooked.
Pour the eggs on top, and bake in the oven for 15 minutes until the top is golden and puffed up.
Cool for 5-10 minutes before cutting and serving.