Combine gelatin and salt and add boiling water.
Stir until gelatin is dissolved.
Peel cucumbers and grate on fine grater into a bowl.
Drain well, saving liquid.
Measure out 1 1/2 cups of grated cucumber pulp and 1/4 cup of the liquid.
Add the cucumber liquid to the gelatin mixture along with the vinegar and onion.
Chill until beginning to set.
Add sour cream and mayonnaise and beat well with rotary beater.
Stir in grated cucumber and 1/4 cup radish slices.
Pour into 9-inch ring mould and chill until firm.
Un-mould on lettuce and garnish with sliced cucumbers and radishes.