Preheat your oven to 375°F (190°C).

In a large mixing bowl, whisk together the eggs. Add a pinch of salt and pepper, mixing well to combine.

In an oven-safe skillet, heat the olive oil over medium heat. Add the chopped red onion and bell peppers, cooking until they soften (about 3-4 minutes).

Stir in the chopped spinach and cook just until wilted (about 2 minutes). Add the cherry tomatoes and remove the skillet from heat.

Pour the egg mixture over the sautéed vegetables. Gently stir to distribute.

Sprinkle the crumbled feta cheese on top (if using).

Transfer the skillet to the preheated oven and bake for 20-25 minutes or until the frittata is puffed and golden on top.

Once done, remove from the oven and let it cool for a few minutes. Slice and serve warm, garnished with fresh basil.