Sauté onion, tomato, leeks, and carrot in 1 tbs of butter until onion turns clear. Add oyster juice and lobster shells. Bring to boil, reduce heat and simmer 40 to 45 min. Remove shells and discard. Remove most of vegetables and set aside. Strain liquid to remove small bits of shell. Make a rue using 4 tbs butter and 4 tbs flour. Cook until it turns light brown. Pour 1/2 of liquid back into oven, whisk well. Add rest of liquid while stirring constantly. Bring to a boil stirring occasionally. Add vegetables, lobster meat and oysters. Simmer 5 to 10 min. Variation: use clams instead of oysters, add celery or bell pepper.