1. Preheat oven to 400°.
2. Heat a 10-inch ovenproof skillet over medium heat. Add oil; swirl to coat. Add onion; sauté 10 minutes or until tender. Add spinach; cook 2 minutes, stirring just until spinach wilts.
3. Combine eggs, salt, and pepper. Pour egg mixture over vegetables in pan; cook until edges begin to set, about 2 minutes. Gently lift edge of egg mixture, tilting pan to allow uncooked egg mixture to come in contact with pan. Cook 2 minutes or until egg mixture is almost set. Sprinkle with cheese.
4. Bake at 400° for 8 minutes or until center is set. Transfer frittata to a serving platter immediately; cut into 6 wedges.