Combine cucumber and vinegar in small bowl; set aside. Combine dill, sugar and mustard in small bowl; mix well.
Heat broiler. Brush bread slices with olive oil on both sides; place on broiler pan. Broil until lightly browned on both sides, about 2-3 minutes, turning once .
Spread about 1 teaspoon mustard mixture on toasted bread slices; top with 2 cucumber slices. Place smoked salmon on each. Garnish with additional fresh dill.
Tip: Cucumbers, mustard mixture and toasted bread can all be prepared ahead. Chill cucumbers and mustard mixture. Assemble crostini just before serving.
