Preheat the oven to 350°F. Lightly coat 6-cup muffin tin with non-stick cooking spray and set aside.

Heat oil in large skillet over medium-high heat. Add onion and pepper, and cook 5 minutes, or until tender. Add spinach and cook 5 minutes, or until spinach begins to wilt. Transfer vegetables to bowl and let cool. Add cheese.

Whisk milk and eggs in large bowl. Season with salt and pepper. Pour eggs over veggies and stir well to combine. Divide the mixture evenly among prepared muffin cups and bake 20 minutes, or until puffed up and set. Serve immediately, or allow them to come to room temperature before covering tightly and placing in refrigerator.