In a medium bowl, whisk together the egg yolk, Dijon mustard, and lemon juice. Using a hand blender, slowly drizzle in the oil, beginning with just a few drops at a time. Once the mixture starts to emulsify and thicken, gradually increase the flow of oil to a thin, steady stream until all the oil is incorporated. Stir in the Seabuckthorn Hot Sauce and season with salt and black pepper to taste.
Bring a saucepan of water to a boil. Carefully lower in the eggs and cook for 9 minutes. Transfer them to an ice bath, then peel and slice into rounds.
Gently toast the rye bread in the oven or in a pan. Lay the rye bread slices on serving plates.
Arrange the sliced eggs evenly on top of the bread, then top with the mayonnaise and the peeled shrimp.
Finish with sprigs of fresh dill and a small handful of watercress. Add a few grinds of freshly ground black pepper if desired.
Enjoy immediately with lemon wedges on the side for squeezing over the shrimp, if desired.
