Preheat your oven to 400 degrees F. Grease an 8-inch oven-safe skillet (such as a cast iron skillet) with butter.
Whisk the egg whites with salt, pepper, and garlic powder until foamy. Add the Parmesan and whisk to incorporate.
Heat the skillet over medium heat, about 3 minutes. Pour the egg mixture into the warm skillet. Allow it to cook for about a minute.
Arrange the tomatoes and scallions on top of the eggs. Keep cooking until the bottom is set, about 2 more minutes, then transfer the skillet to the oven to finish cooking for about 10 more minutes. The frittata is done when puffy and golden and a toothpick inserted in its center comes out clean.
Carefully loosen the edges of the frittata with a spatula, slice and serve.
While you certainly want the frittata to be fully cooked, you don't want to overcook it, or it will become dry and grainy. So make sure to take it out of the oven as soon as it's done - when it becomes puffy and golden and a toothpick inserted in its center comes out clean.