01 - Preheat your oven to 375°F.
02 - In a medium oven-safe skillet, heat olive oil over medium heat. Add onion and bell pepper; sauté for 3–4 minutes until softened.
03 - Stir in garlic and cook for 1 minute. Add spinach and cook for another 1–2 minutes until wilted.
04 - Add cooked lentils and mix until evenly combined.
05 - In a large bowl, whisk together eggs, milk, oregano, salt, and pepper. Stir in half the parsley.
06 - Pour the egg mixture evenly over the vegetables and lentils in the skillet. Gently shake or stir to distribute.
07 - Sprinkle crumbled feta evenly on top.
08 - Cook on the stovetop over low heat for 2–3 minutes until the edges begin to set.
09 - Transfer the skillet to the oven and bake for 12–15 minutes, or until the frittata is set in the center and lightly golden.
10 - Remove from the oven, let cool slightly, and garnish with remaining parsley. Slice and serve warm or at room temperature.