1. Preheat the oven to 375°F (190°C).

2. Spray a pie pan with olive oil and arrange the sweet potato slices to cover the bottom and sides of the pan, forming a crust. You can cut the rounds in half to fit the sides.

3. Spray the sweet potato rounds lightly with olive oil. Bake the crust in the oven for 20 minutes.

4. While the crust is baking, sauté the diced red bell pepper and onion in a pan over medium heat with a little salt and pepper for about 5 minutes, until softened.

5. Add the frozen or fresh spinach to the pan and continue to cook until wilted and well combined with the vegetables. If using frozen spinach, you can thaw it first to remove excess moisture.

6. In a bowl, whisk together the eggs, egg whites, and milk. Season with salt and pepper.

7. Once the sweet potato crust is done baking, remove it from the oven and adjust any pieces as needed.]

8. Spread the spinach and vegetable mixture evenly over the sweet potato crust.

9. Pour the egg mixture on top and sprinkle the feta cheese over the quiche.

10. Return the quiche to the oven and bake for an additional 40 minutes, or until the eggs are fully set and the quiche is golden brown.

11. Let the quiche cool slightly before serving. Enjoy!