For the beet gravlax: Blitz the beet to a smooth puree in a food processor, then transfer to a bowl. Add the sugar, salt, pepper, honey, mustard and dill; mix to blend.
Roll out a large piece of plastic wrap and sprinkle with half of the beet mixture. Place the salmon, skin-side down, on the beet mixture and top with the remaining mixture. Wrap the salmon tightly in the plastic wrap, adding an extra layer if needed.
Place the wrapped parcel in a high-sided dish and weight it down with a plate and some cans, or whatever heavy item you have at hand.
Leave the salmon to cure in the fridge for 5 days, turning the salmon halfway through. The dish will fill with juices but don't be too concerned about draining them until you are ready to serve.
For the pickled cucumbers: In a bowl mix together the sugar and vinegar until the sugar has dissolved. Mix in the salt and shallots and then add the cucumber slices. Cover with plastic and leave in the fridge for at least 2 hours and up to overnight.
For the dill creme fraiche: Mix the creme fraiche, dill, lemon juice and a good grinding of black pepper in a bowl. Cover with plastic and leave in the fridge for at least 2 hours and up to overnight.
To serve: Unwrap the salmon and rinse off the cure under cold running water. Pat dry, then slice thinly with a large knife.
Arrange crispbreads on serving plates. Top each with the slices of gravlax, dollops of dill creme fraiche, and a small mound of pickled cucumbers. Garnish with watercress and dill.