Preheat oven to 375°F. Spray an 8-inch pie pan with nonstick spray or line it with nonstick foil.
Press 2cups potatoes shredded evenly into the pan, pressing up the sides to form a crust.
Bake the potato crust for 10 minutes. Remove from the oven.
Sprinkle half of the 3/4cup feta cheese crumbles into the bottom of the hot crust and set aside.
Heat 1tablespoon olive oil in a small skillet over medium heat. Add 1cup grape tomatoes (halved) and 3cups baby spinach. Cook, stirring, until the spinach is wilted and the tomatoes are slightly softened, about 2–3 minutes.
Transfer the tomato-and-spinach mixture onto the feta in the crust, spreading it evenly. If there is excess liquid in the skillet, drain it before adding.
In a bowl, whisk together 2eggs, 2egg whites, 1/4cup skim milk, 1/2teaspoon kosher salt, and 1/4teaspoon ground black pepper until combined.
Pour the egg mixture evenly over the spinach and tomatoes in the crust. Sprinkle the remaining feta over the top.
Bake for about 30 minutes, or until the filling is set and lightly browned.
Remove from the oven and let rest for 10 minutes before slicing and serving.