Cut several thin slices of cucumber for garnish; soak in ice water until needed. Chop remaining cucumber and onion. Saute in butter until wilted but not browned. Process sauteed vegetables with 2-3 drops of green food coloring in blender or food processor until smooth. Return to saucepan. Add chicken stock, lemon juice, dill (crush seeds lightly) and salt; simmer 5 minutes. Chill 6 hours. Just before serving, stir in sour cream. Garnish with crisp cucumber slices and sprinkle with fresh or dried dill.