Peel cucumber and slice in half lengthwise. Remove seeds with a small spoon. Place cucumber in a food processor and pulse 5 times, until minced.
Place minced cucumber in small colander over glass bowl. Sprinkle with salt, toss, and allow to drain over bowl for one hour. Spread drained cucumber on dinner plate and pat with paper towel to remove excess moisture. Place cucumber in small glass bowl.
Add onion and cream cheese to cucumber and stir well with spatula. Add hot sauce, food coloring. Season to taste with additional salt and pepper.
Cut rounds out of bread slices with cookie or biscuit cutter. Spread small amount of mayonnaise on bread rounds. Spread cucumber mixture on half of rounds and top with another round. Garnish with sprig of parsley or watercress. Store covered in refrigerator. Serve chilled.