Peel and finely dice the shallots. Peel, halve, core and dice the cucumber. Heat half of the butter and sauté the shallots and diced cucumber in it. Add wine and cream and simmer for 5 minutes. Stir cornflour in a little water until smooth and thicken the vegetables with it. Season to taste with mustard, salt, pepper and sugar. Wash the dill, dab dry, put something aside for garnishing.
Chop the rest finely and add to the sauce. Heat the remaining butter and toss the crab meat in it. Lightly toast the baguette. Wash and halve the tomatoes. Spread the sauce on 4 plates and place 3 baguette slices on each plate. Arrange the crabs on top. Garnish with dill and tomato halves