Sauté cucumber and onion in butter in a large Dutch oven until tender. Add flour, stirring until smooth. Cook 5 minutes, stirring constantly. Gradually add fish stock; cook over medium heat, stirring constantly, until thickened and bubbly. Stir in half-and-half, crab meat, salt, and pepper. Remove from heat; cover and chill thoroughly.
Stir in wine just before serving. Serve chilled in individual soup bowls. Garnish with cucumber slices, and serve with French bread, if desired.