Preheat your oven to 375°F and lightly grease a small 6-inch oven-safe skillet with a touch of oil.
Peel the sweet potato and dice it into small half-inch cubes to ensure they cook through quickly.
In a separate non-stick skillet over medium-high heat, brown the ground beef until fully cooked and no longer pink, then drain any excess fat and set aside to cool slightly.
In the oven-safe skillet, heat the olive oil over medium heat and sauté the sweet potato cubes for 8-10 minutes until tender and golden brown.
Add the diced red bell pepper to the skillet and sauté for another 2-3 minutes until softened.
Stir in the baby spinach and cook just until wilted, then spread the vegetables evenly across the bottom of the pan.
In a medium mixing bowl, whisk together the eggs, sea salt, black pepper, and dried oregano until well combined.
Fold the cooked ground beef directly into the egg mixture so it is fully incorporated.
Pour the beef and egg mixture over the vegetables in the skillet, ensuring the beef is distributed evenly throughout the dish.
Sprinkle the crumbled feta cheese over the top and bake for 15-18 minutes until the eggs are set and the edges are slightly puffed.
Allow the frittata to rest for 5 minutes before slicing into wedges and serving.