Preheat the oven to 400°F (200°C). Heat 1 tbsp olive oil in a 10-inch oven-safe skillet over medium heat. Add the sweet potato and onion, season with salt, and cook 6-8 minutes until tender and lightly browned.

Add the spinach in handfuls, stirring until wilted, about 1 minute. Remove the pan from heat and spread the vegetables evenly.

Whisk the eggs, feta, salt, black pepper, and red pepper flakes together in a bowl until combined.

Drizzle the remaining 1 tbsp olive oil over the vegetables. Pour in the egg mixture, shake gently, and cook over medium heat 3-4 minutes until edges set. Transfer the skillet to the oven and bake 12-15 minutes, until puffed and barely jiggly in the center.

Run a spatula around the edge, slide the frittata onto a board, slice into wedges, and serve warm or at room temperature.