by Amanda Haas
Preheat oven to 400°F.
Heat oil in a 10-inch ovenproof skillet over medium heat. Add onion; sauté 10 minutes or until tender. Add spinach; cook 2 minutes, stirring just until spinach wilts.
Bake at 400°F for 8 minutes or until center is set. Transfer frittata to a serving platter immediately; cut into 4 wedges.