Spinach and Feta Quiche with Sweet Potato Crust

Cooking spray

medium sweet potatoes, peeled and cut into 1/8-in.-thick slices

canola oil

sliced onion

bag fresh baby spinach

1% low-fat milk

kosher salt

freshly ground black pepper

crushed red pepper

large eggs

large egg whites

feta cheese, crumbled (about 1/3 cup)