Whisk eggs with water; season with salt and pepper. Set aside. Place ham slices overlapping slightly in a 25 cm (10-inch) non-stick skillet. Spread pesto over ham slices. Heat skillet over medium heat. Pour egg mixture over ham and pesto in skillet. As mixture sets at the edges, with spatula, gently lift cooked portion to allow uncooked egg to flow underneath. Cook until bottom is set and top is almost set. Turn off heat. Sprinkle tomato over frittata; cover skillet to heat tomato. Loosen edges and slide frittata onto a warm plate. Slice into four wedges and serve.