Preheat your oven to 375°F (190°C).
Prepare your vegetables: Dice the onion, bell pepper, and tomato. Chop the spinach roughly. If using olives or sun-dried tomatoes, chop them as well.
Sauté the vegetables: In an oven-safe skillet, heat olive oil over medium heat. Add diced onion and bell pepper, cooking for 3-4 minutes until softened. Add the tomato and spinach, cooking for another 2-3 minutes until the spinach wilts.
Prepare the egg mixture: In a large mixing bowl, crack the eggs and whisk together. Add salt, pepper, oregano, and fresh basil. Stir in the crumbled feta cheese.
Combine eggs and vegetables: Pour the egg mixture over the sautéed vegetables in the skillet. Stir gently to combine. Let the frittata cook for 2-3 minutes on the stove until the edges start to set.
Bake: Transfer the skillet to the oven and bake for 10-12 minutes, or until the frittata is fully set in the center and lightly golden on top.
Serve: Remove from the oven, garnish with additional fresh herbs or crumbled cheese, and slice into wedges. Serve warm or at room temperature.