Line baking tray with baking paper. Spread french fries on top. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 180 °C/ gas: level 3) for approx. 20 minutes, turn once after 14 minutes.
Heat the sausages in hot water for a few minutes. Mix mustard and mayonnaise. Drain the cucumbers. Take the French fries out of the oven. Warm the rolls in the oven for 1-2 minutes. Season the French fries with salt.
Drain the sausages. Cut the roll lengthwise. First add the sausages, then the cucumber to the roll. Spread mustard sauce and ketchup on top and sprinkle with fried onions. Serve together with the French fries.
Garnish with salad.