Step 1Place the gelatin and salt in a large bowl. Add the boiling water and stir until completely dissolved, 2 minutes. Stir in the cold water and lemon juice. Refrigerate until slightly thickened (it should have the consistency of unbeaten egg whites), about 1 1/4 hours.
Step 2Mix the mayonnaise and sour cream in a small bowl until well blended. Stir into the thickened gelatin. Refrigerate until thickened (a spoon drawn through leaves a definite impression), about 15 minutes. Stir in the cucumbers, onion and dill. Pour into a 5-cup mold.
Step 3Refrigerate until firm, 4 hours. Unmold. Garnish as desired.