Preheat the oven to 350°, then peel and slice sweet potatoes. Coat a pie dish with cooking spray, then fill the bottom of the dish with layers of sweet potato slices. Cut the sweet potato rounds in half and fill around the edges of the pie dish; round side facing up. Bake for 20 minutes. Turn heat up to 375°.

For the filling, heat a skillet over medium heat. Add 1 tbsp olive oil and the sliced onion, cooked till caramelized then add the spinach; sauté 3 minutes. Remove from heat.

Combine milk and next 5 ingredients in a bowl; stir with a whisk.

Arrange spinach mixture in crust; pour egg mixture over spinach. Sprinkle with feta. Bake at 375° for 35 minutes. Let the quiche rest for 5 minutes.