Preheat oven to 375ºF. Mist a 12-cup muffin tin with nonstick cooking spray. Cut each slice of ham in half and press 1 piece into each cup.
Warm oil in a skillet over medium-high heat. Cook potato, stirring, until softened and golden, about 5 minutes. Add onion and bell pepper and cook, stirring, until softened, 3 to 5 minutes longer. Season with salt and divide among muffin cups.
In a large measuring cup, whisk together eggs, milk and 1/4 tsp. salt. Pour egg mixture into muffin cups so each one is about half full. Bake until eggs are cooked through, about 15 minutes. Let stand on a wire cooling rack for 5 minutes. Remove frittatas from muffin tin and serve warm, or let cool completely and serve at room temperature.