Wash vegetables. Peel the carrots, potatoes and kohlrabi and then dice them. Cut the leek into rings. Heat the oil in a saucepan. Sauté the leek, carrots and kohlrabi. Add the tomato paste, gently fry and then pour in the vegetable stock. Add the potato cubes, season with pepper and simmer for 15 minutes. Sprinkle with basil.