Cut off the stem ends and green parts of leeks, then wash thoroughly and cut into 3 cm slices. Soak in hot water for 5 to 10 minutes, then drain and discard the water. In a non-aluminum pan, combine with the tomatoes or tomato juice, stock or broth, onion, celery, parsley, thyme or oregano, 3 tablespoons butter or oil, and salt and pepper. Simmer until tender (approximately 20 minutes), adding the lemon juice during the last 5 minutes. Serve warm or cold.