Preheat oven to 375 degrees. Brush a 9-inch square baking dish with oil. Bring water to a simmer in a medium skillet over high heat. Add leeks, garlic, and potatoes; season with 1/2 teaspoon salt. Reduce heat to medium-low, and cook, covered, until potatoes are just tender, 8 to 10 minutes. Let cool.
Whisk together eggs, whites, 1/2 teaspoon salt, and the cayenne. Fold in potato mixture and ricotta. Mix in 1/4 cup Parmesan cheese. Pour mixture into dish, and sprinkle with remaining Parmesan. Bake until edges are set, about 12 minutes. Reduce oven temperature to 325 degrees, and bake until set and a knife inserted into the center comes out clean, 20 to 25 minutes. Let cool for 10 minutes. Cut into 8 squares.
Storage: Frittata can be refrigerated for up to 2 days.