For the potato salad: Cook the potatoes, with their skin on, over gentle heat, until just tender. Meanwhile, make the dressing by mixing lemon juice, olive oil, dill, capers, salt and pepper. Strain the potatoes and set aside for a minute to steam dry and cool slightly – but not entirely. Slice the potatoes thinly. Fold warm potatoes into the dressing. Set aside.
For the dressing: In a small bowl, combine crème fraîche, horseradish, water, and mix well. Add salt and pepper to taste.
Arrange slices of potatoes, eggs, red onion and Gravlax neatly on each crisp. Drizzle with horseradish dressing and top of with caviar or salmon roe (if using).