Melt the fat in a pot. Stir in flour, sweat briefly. Add milk and stock little by little while stirring. Bring to the boil, simmer for 6-8 minutes, remove from heat. Season to taste with salt, pepper, nutmeg and 1 dash of lemon juice.
Peel the kohlrabi and slice into thin slices. Layer kohlrabi in an ovenproof gratin dish (approx. 9 cm Ø; 5 cm high), pour half of the béchamel sauce over them and sprinkle with cheese. In a preheated oven (electric cooker: 200 °C/ circulating air: 175 °C/ gas: see
manufacturer) cook for about 25 minutes.
Meanwhile wash the salmon, dab dry and season with salt, pepper and 1 dash of lemon juice. Heat 1 tsp. oil in a coated pan. Fry the fish for 2-3 minutes on each side. Wash the chervil, shake dry, pluck off the leaves and, except for a few for garnishing, cut into small pieces.
Clean and wash the cucumber and slice it lengthwise into thin strips. Mix vinegar, salt, pepper and sugar. Slowly fold in 2 teaspoons of oil. Heat the remaining béchamel sauce again. Remove from heat and stir in the chopped chervil.
Mix cucumber and vinaigrette. Remove the kohlrabigratin from the oven and arrange on a plate with salmon, cucumber salad and chervil sauce. Garnish with chervil.