Peel, wash and quarter sweet potatoes and cook them in boiling salted water for 10-15 minutes until soft. In the meantime, wash and clean the cucumbers, peel the skin with a pestle ripper in strips. Cut the cucumbers into 4-5 pieces each and hollow them out with a ball cutter.
Heat the oil in a large pan, fry the cucumbers and bake in a preheated oven (electric cooker: 150 °C/ convection oven: 125 °C/ gas: level 1) for approx. 5 minutes. Peel and halve the onion and cut into fine slices.
Wash the chives, dab dry and cut into fine rings. Heat the fat in a pot and sauté the flour in it. Deglaze with stock while stirring. Season with salt, pepper, mustard and cream and simmer for about 5 minutes, add chives.
Drain the sweet potatoes, let them evaporate and press them through a potato press. Mix with olive oil and season with salt and pepper. Put the puree into a piping bag and fill the cucumbers with it.
Arrange gherkins with mustard sauce and garnish with onion rings and marjoram.