Peel and finely chop the onion. Clean and peel the kohlrabis. Cut 1 kohlrabi into large cubes. Peel and wash the potatoes and cut them into large cubes. Heat the fat in a saucepan, fry the onion, kohlrabi cubes and potatoes in it at low heat.
Deglaze with broth and simmer for about 15 minutes. Add cream and bring to the boil. Season to taste with salt, cayenne pepper and lemon juice. Puree the soup with a chopping stick. In the meantime cut the remaining kohlrabi into sticks and cook in boiling salted water for about 8 minutes.
Cut toast into cubes. Heat oil in a pan. Fry the toast cubes until golden brown and remove. Cut cress from the bed with scissors and put aside about 1/3 for sprinkling. Remove approx. 200 ml kohlrabi cream soup and puree with the cress.
Arrange the rest of the soup in soup plates. Drain the kohlrabi sticks, spread them in the soup and drizzle cress puree over them. Sprinkle with toast cubes and cress.