Preheat oven to 400.
Prepare Spinach: If using frozen, thaw spinach in a microwave-safe bowl for about 5 minutes; set aside to cool and then drain and squeeze dry. If using fresh, stir fry baby spinach in a skillet with a teaspoon of olive oil.
Prepare Sweet Potato Crust: Peel and slice sweet potato with a mandolin to 1/8-inch slices. Spray pie dish with olive oil mister. Layer sweet potatoes, overlapping around bottom of pie dish and up the sides. Spray again with olive oil. Bake at 400 for 15 minutes; set aside until cool and lower oven temp to 350. (Potato slices shrink and move a little while baking, you may have to rearrange them, but they don't have to be perfect.)
While sweet potato is baking, add a tablespoon of olive oil to a cast iron skillet and cook chopped onion about 5 minutes until translucent and slightly browned.
Add spinach to onion, and stir to heat through. Then add remaining ingredients to spinach mixture, except for eggs and milk. Stir to combine and spoon into pie dish carefully on top of sweet potato crust. Pour beaten eggs on top of spinach mixture.
Bake at 350 for 20 to 25 minutes or until firm in center.
I love using a sweet potato "crust" for quiche--it keeps this dish in the "real food" category and provides a slight natural sweetness that balances the savory flavors perfectly. Pro tip: I usually make two sweet potato crust quiches at a time as long as I have the mandolin out; it makes for easy leftovers for breakfasts or a quick weeknight meal.
Quiche is also very forgiving--any combination of sauteed veggies will work as long as they aren't too watery to allow the eggs to bind it all together in a delicious, one-dish breakfast or dinner! You can also feel free to adjust the seasonings or the cheeses to your tastes.