Whisk eggs with water, parsley and oregano; season with pepper. Set aside. Spray a 10 inch (25 cm) non stick skillet with cooking spray. Heat skillet over medium heat. Saute leeks, sweet potato and prosciutto for about 5 minutes or until vegetables are cooked. Pour in egg mixture; 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.
Sprinkle with cheese. Place under a preheated broiler* for 1 to 2 minutes to melt cheese or cover skillet with a lid and cook another minute. Loosen edges and slide frittata onto a warm plate.
*To ovenproof handle, wrap with aluminum foil.