Preheat oven to 350 degrees F. Grease 10-inch quiche dish or 9 1/2-inch deep-dish pie plate.
Cut off roots and trim dark green tops from leeks. Discard any tough outer leaves. Cut each leek lengthwise in half, then crosswise into 1/4-inch-wide slices. Rinse leeks thoroughly in large bowl of cold water, swishing to remove sand. With hands, transfer leeks to colander to drain, leaving sand in bottom of bowl. Repeat process, changing water several times, until all sand is removed. Shake colander several times to remove excess water from leeks. Cut leeks into 3-inch long pieces.
In 12-inch nonstick skillet, heat oil on medium 1 minute. Add leeks and 1/4 teaspoon salt, and cook 12 to 14 minutes or until leeks are tender and browned, stirring frequently. Transfer leeks to prepared quiche dish and spread evenly.
Meanwhile, in bowl, with wire whisk, beat eggs, milk, cornstarch, 1/4 teaspoon salt, and 1/4 teaspoon coarsely ground black pepper until well blended.
Pour egg mixture over leeks in dish. Sprinkle with Gruyère. Bake 30 to 35 minutes or until knife inserted in center comes out clean. Cool on wire rack for 5 minutes.