Preheat oven to 350°F. Coat a 9-inch pie plate with cooking spray. Layer sweet potatoes in slightly overlapping concentric circles on bottom and upsides of plate, cutting slices in half to fit (rounded side up) around the sides. Coat potatoes with cooking spray. Bake in preheated oven for 20 minutes or until potatoes are slightly tender. Place pan on a wire rack. Increase oven temperature to 375°F. Heat a large nonstick skillet over medium. Add oil and onion; saute 3 minutes. Add spinach; saute 3 minutes. remove from head; cool. Combine milk and next 5 ingredients (through egg whites) in a medium bowl; stir with a whisk. Arrange spinach mixture in crust; pour egg mixture over spinach. Sprinkle with feta. Bake at 375°F for 35 minutes or until egg mixture is set. Let stand 5 minutes; cut into 8 wedges.