Peel the cucumber, reserve a 2-in. length for garnish, slice the rest. Melt the butter in a deep pan and cook the onions gently, without browning, for 10 min. Stir in the flour, then the stock and bring to boiling-point. Add the sliced cucumber and cook till tender. Sieve through a nylon sieve. Season and add lemon juice to taste. Cut the 2-in. piece of cucumber into