Cut part of the cucumber into small pieces and slice the other part evenly to use for garnish. Mash the eggs, reserve 1/3 cup for garnish. Chop the green onions. Reserve 1/3 for garnish. Combine the rest with the eggs.

Put water and lemon juice into a heavy saucepan. Sprinkle with gelatin. Allow it to soften for 5 minutes. Heat at low temperature. Put 3/4 cup of mayonnaise into a bowl. Add 1/2 of the sour cream, 1 tsp. mustard. Add gelatin and blend. Combine with eggs and cucumber mixture. Put into a spring form pan or a quiche dish, greased. Refrigerate for 2 hours.

Unmold mousse into serving platter lined with lettuce leaves or parsley. Spread top with other half of sour cream. Arrange caviar in the centre. Ring with a border of chopped eggs, green onion and cucumber. Serve with pumpernickel bread.