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