1. Discard bottom 1 1/2 to 2 inches of thick stems from watercress; coarsely chop remaining leaves and stems. Core lettuce, rinse well, shake dry, and coarsely chop. 2. Heat butter in a heavy-bottomed 3-quart saucepan and add watercress, lettuce, leeks, celery leaves, 1 T of mint, and salt. Cover and cook over medium-low heat until just tender, about 10 minutes. 3. Add stock and water, bring to a boil, and simmer, covered, for 30 minutes. 4. In a small bowl, whisk eggs with lemon juice to blend, then slowly whisk in 1/2 C of soup. Slowly whisk this mixture into soup pot and cook, stirring, over low heat until slightly thickened, about 1 minute. Do not let soup boil. Add pepper and adjust seasonings. Add rice and garnish with remaining mint. Martha Stewart Living/Feb. & March/94 Scanned & fixed by Di and Gary Posted to MM-Recipes Digest V4 #035 by John Merkel on Feb 3, 1997.