In a large skillet, heat oil and sauté onions over moderate eat for about 5 minutes. Add leeks and, stirring frequently, continue sauteing another 15 minutes or until onions are golden and leeks are limp. Remove from heat and set aside. Preheat oven to 350º. In a mixing bowl, combine beaten eggs with 2 T. of the parsley, the tarragon, pepper, and feta. Stir in the onion and leek sauté until well combined. Oil a 10-inch tart pan and cover bottom generously with breadcrumbs. Gently pour onion mixture over breadcrumbs. Line the outside edge with tomato slices; lightly sprinkle remaining breadcrumbs over the top and the remaining parsley in the center. Bake 40-45 minutes or until the mixture is set on the top is golden. Let stand to cool at least 5-10 minutes before serving.