Preheat the oven to 350ºF. Spray a 12-cup muffin tin with nonstick cooking spray.
In a bowl, lightly beat the eggs. Season with salt and pepper.
Divide the spinach between the 12 cups, followed by the onion and the ham. Add the whisked eggs, dividing evenly between the 12 cups. Top each cup with some of the cheese.
Bake in the oven until puffed and cooked through, about 20 minutes.
Notes
Nutrition information provided as an estimate only. Various brands and products can change the counts. Any nutritional information should be used as a general guide.