Preheat the oven to 350 degrees F (175 degrees C). Spray a 12-cup muffin tin with nonstick cooking spray.
Place a thin layer of potato slices in the bottom of each muffin cup. Add a little butter on top.
Bake in the preheated oven for 5 minutes.
Mix together eggs, bell pepper, tomato, spinach, ham, and onion in a large bowl. Ladle egg mixture over warm potatoes. Top each muffin cup with a mozzarella cube.
Continue baking until eggs are set, about 20 minutes.
