Step 1Lightly beat eggs with salt and pepper to taste.
Step 2Heat oil in 10-inch nonstick, ovenproof skillet over medium heat. Add leek and saute until just starting to soften, about 2 minutes. Add prosciutto and saute until leek is soft, 1 to 2 minutes. Remove half of leek mixture and set aside.
Step 3Add potatoes and rosemary to skillet and toss to heat potatoes, 1 minute. Reduce heat to medium-low and add eggs. Do not stir. Cook just until bottom of eggs is set, 3 to 4 minutes. Top should still be wet.
Step 4Sprinkle with cheese and reserved leek mixture. Broil 4 inches from heat source until frittata is golden and sizzling, about 2 minutes.
Step 5Invert frittata onto cutting board, sprinkle with tomato and parsley, and cut into wedges.