Preheat oven to 400°F (200°C) and grease bottom and sides of a 9x9 round baking pan with oil or butter.
In a large mixing bowl, whisk egg whites with salt and pepper.
Add chopped spinach, diced tomatoes, sliced mushrooms and sliced onion. Optionally, you can add extra ingredients such as cottage cheese, crumbled feta or bacon. Gently whisk to combine, without overmixing.
Pour the mixture to the prepared baking dish, distributing the vegetables evenly.
Bake in hot oven for 20 - 25 minutes, or until a toothpick inserted into the center comes out clean. Do not worry if the frittata puffs up in the oven, it will deflate as it cools.
Serve immediately or let cool and store refrigerated in an airtight container for up to 4 days.