Peel cucumber and onion and dice finely. Wash the salmon, dab dry and cut into slices. Heat 1 tablespoon of oil in a frying pan, fry the salmon briefly. Add 300 ml stock and white wine and let it simmer at low heat for about 10 minutes. Wash the dill, dab dry and chop finely, except for a little to garnish.
Add lemon zest and half of the dill to the fish. In the meantime, heat 1 tablespoon of oil in a pan, fry the onion and cucumber cubes, add 2-3 tablespoons of stock and continue to steam for about 2 minutes. Season with salt and pepper. Remove salmon and keep warm. Add crème fraîche and mustard to the fish stock, mix well and bring to the boil. Stir starch and 2 tablespoons of water until smooth. Take the sauce off the stove, stir in the starch. Bring to the boil again while stirring and let it boil for at least 1 minute. Stir in the remaining dill, season the sauce with salt, pepper, sugar and lemon juice. Arrange salmon, cucumber cubes and sauce on plates.
Stir starch and 2 tablespoons of water until smooth. Take the sauce off the stove, stir in the starch. Bring to the boil again while stirring and let it boil for at least 1 minute. Stir in the remaining dill, season the sauce with salt, pepper, sugar and lemon juice. Arrange salmon, cucumber cubes and sauce on plates. Garnish with dill. Serve with boiled potatoes