Peel cucumber and slice in half lengthwise; remove seeds with a small spoon. Place cucumber in a food processor and pulse about 5 times, until cucumber is minced. Place cucumber into a small glass mixing bowl. Place onion in the food processor, pulsing until it is finely chopped. Add onion to cucumber. Add cream cheese and stir well with a spatula. Add salt, hot sauce, and food coloring. With a round cookie or biscuit cutter, cut rounds out of bread slices. Spread a small amount of mayonnaise on bread rounds. Spread cheese mixture on half the rounds and top with another round. Garnish with a sprig of parsley or watercress.