01 - Preheat the oven to 375°F (190°C).
02 - Heat olive oil in an oven-safe skillet over medium heat. Add diced onion and cook for 2 minutes until beginning to soften. Stir in chopped kale and sauté until wilted and tender, about 2 minutes more.
03 - In a large bowl, whisk eggs thoroughly with salt and black pepper.
04 - Fold in roasted sweet potatoes, sautéed onion and kale mixture, and crumbled feta cheese. Mix gently until evenly integrated.
05 - Pour the egg mixture into the same oven-safe skillet. Transfer skillet to the oven and bake for 20 minutes until the frittata is set in the center and lightly golden on top.
06 - Remove from oven and allow to cool for 5 minutes before slicing and serving warm.