1. Preheat oven to 350°.
2. Coat 6 muffin cups with cooking spray. Heat a medium nonstick skillet over medium-high heat. Add oil; swirl to coat. Add onion; sauté 3 minutes or until almost tender. Add spinach; sauté 2 minutes or just until spinach begins to wilt, stirring constantly. Transfer spinach mixture to a small bowl; cool 3 minutes. Stir in cheese.
3. Combine milk and remaining ingredients, stirring with a whisk until blended. Stir in cheese mixture. Divide mixture evenly among prepared muffin cups. Bake at 350° for 20 minutes or until puffed and set. (Quiches will deflate slightly as they cool.) Serve warm.