Preheat oven to 350 degrees. Grease 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 in large bowl of water, swishing to remove sand. Transfer leeks to colander and drain. Repeat until all sand is removed.
In a 12 inch skillet, heat oil on med for 1 minute Add leeks and 1/4 tsp salt; cook 12-14 min or until tender & brown, stirring frequently. Transfer leeks to prepared quiche dish.
Meanwhile, in bowl, with wire whisk, beat eggs, milk, cornstarch, 1/4 tsp salt & 1/4 tsp black pepper until well blended.
Pour egg mixture over leeks in dish. Sprinkle with gruyere. Bake 30-35 minute Cool on wire rack for 5 minute.