Preheat oven to 375°F and grease a 9-inch pie dish with a touch of oil.
Peel and slice the sweet potato into very thin rounds, then layer them across the bottom and sides of the dish to form a crust.
Bake the sweet potato crust for 15 minutes until the edges are slightly crisp.
While the crust bakes, sauté the spinach in olive oil over medium heat until wilted, then squeeze out all excess moisture.
In a large bowl, whisk together the eggs, egg whites, Greek yogurt, sea salt, black pepper, and garlic powder until completely smooth.
Spread the wilted spinach and crumbled feta evenly over the pre-baked sweet potato crust.
Pour the egg and yogurt mixture over the filling, ensuring it is evenly distributed.
Bake for 30 to 35 minutes until the custard is set and the top is beautifully golden brown.