Add the watercress and lemon juice to the blender. Blend until smooth. Place the watercress in cheesecloth and squeeze out excess water. Set aside. Add the egg yolk and mustard to the blender and blend for 15 seconds. Slowly drizzle in the oil until the mixture has the consistency of mayonnaise. Season with salt and pepper. Fold the pureed watercress into the mayonnaise. Reseason with hot pepper sauce and lemon juice.
Spread watercress mayonnaise on 18 slices of pumpernickel bread. Top with 2 ounces of smoked salmon. Top with 1 slice of black radish. Place another slice of bread on top.
*RAW EGG WARNING
Food Network Kitchens suggest caution in consuming raw and lightly-cooked eggs due to the slight risk of Salmonella or other food-borne illness. To reduce this risk, we recommend you use only fresh, properly-refrigerated, clean, grade A or AA eggs with intact shells, and avoid contact between the yolks or whites and the shell.