Preheat oven to 450°. Remove and discard root ends and dark green tops of leeks. Cut in half lengthwise, and rinse thoroughly under cold running water to remove grit and sand. Thinly slice leeks.
Sauté leeks in 1 Tbsp. hot oil in a 10-inch ovenproof skillet over medium-high heat 4 to 5 minutes or until tender. Remove from skillet. Sauté potato in remaining 1 Tbsp. oil in skillet 3 to 4 minutes or until golden. Add squash, and sauté 10 minutes. Stir in leeks, parsley, thyme, pepper, and 1/2 tsp. salt until blended.
Process eggs, milk, 1/2 cup Parmesan cheese, 1/4 cup water and remaining 1/4 tsp. salt in a blender until blended; pour over leek mixture in skillet. Cook over medium heat, without stirring, 2 minutes or until edges of frittata are set. (Edges should appear firm when pan is gently shaken; the top layer should appear wet.) Sprinkle with remaining 1/4 cup cheese.
Bake at 450° for 10 to 12 minutes or until center is set.