Boil spaghetti in salted water until al dente. Strain and toss them in some olive oil.
Peel kohlrabi and dice them into 5 mm cubes. Place cubes in a pot, cover with water, add salt, pepper and 1/2 tbsp. butter and boil  firm to the bite.
Season char fillets with salt and pepper, place them upon the cabbage, cover and steam for about 5 minutes.
Remove kohlrabi and fish and arrange them on warmed plates. Mix kohlrabi-fond with lemon juice and 1/2 tbsp. butter. Season with salt and pepper. Stir in whipped cream. Cover fish with the sauce and serve with spaghetti.