Preheat your oven to 375°F (190°C).

Heat the olive oil in a non-stick skillet over medium heat. Add the chopped onion and garlic, cooking for 2-3 minutes until fragrant and translucent.

Add bell peppers and other veggies, cooking for about 5 minutes until slightly softened but still vibrant. Season lightly with salt and pepper. Remove from heat and let cool slightly.

In a large mixing bowl, whisk the egg whites until combined and slightly frothy. Add a pinch of salt, pepper, and dried herbs if using.

Gently fold the sautéed veggies and shredded cheese into the egg whites, being careful not to overmix.

Lightly grease your baking dish with olive oil or non-stick spray. Pour the egg white mixture evenly into the dish and smooth the top with a spatula.

Bake in the preheated oven for 20-25 minutes until the frittata puffs up and is set in the center. A toothpick inserted should come out clean.

Let the frittata cool for 5 minutes before slicing into portions.

Serve the frittata warm, sliced into squares. It pairs nicely with a light salad or some fresh fruit for a complete meal. You can also top it with a dollop of yogurt or a sprinkle of extra herbs for added flavor.

Once cooled, store any leftover frittata in an airtight container in the refrigerator. It will keep well for up to 4-5 days. You can also freeze portions by wrapping them tightly in plastic wrap and placing them in a freezer-safe container. Just thaw and reheat when you're ready to enjoy!