Peel and wash the kohlrabi and potatoes and cut into small cubes. Peel and dice the shallots.
Heat the butter and sauté the vegetables and shallots. Deglaze with soup and 1 tbsp. lemon juice. Cover and simmer until the vegetables are firm to the bite.
Wash the rocket, shake dry, remove the coarse stems and cut the leaves into strips.
Drain the vegetable soup and keep it. Mix 1 tbsp. lemon juice, 5 tbsp. soup, salt, pepper and 3 tbsp. oil and fold into the vegetable mixture. Add the rocket and season with salt and pepper.
Salt and pepper the fillets. Heat 2 tbsp. oil in a pan and fry the fish on both sides.
Serve with the tartare and garnish with dill and pepper.