Lightly grease skillet with olive oil and set aside.
Heat olive oil in a skillet over medium heat. Add the cherry tomatoes and cook until they begin to soften and release their juices. Add the garlic and cook briefly until fragrant.
Add the spinach and cook for 1 to 2 minutes until it wilts. Spread the cooked vegetables evenly across the bottom of the prepared skillet or baking dish.
In a large bowl, whisk the egg whites with salt, black pepper, and Italian seasoning until slightly frothy. Pour the egg whites evenly over the vegetables.
Sprinkle the crumbled feta cheese over the top and gently press it into the egg mixture so it stays evenly distributed.
Cook for 20 to 25 minutes, or until the frittata is puffed, set in the center, and lightly golden around the edges.
Let it cool for a few minutes, and garnish with fresh basil or parsley. Slice and serve warm or at room temperature.
