Brush slices of bread on both sides with butter and roast in a hot grill pan on both sides until golden brown.
Wash the arugula and pat dry. Peel onions and cut into rings.
Heat some oil in a frying pan. Sauté the onion rings in the pan until transparent and sprinkle with sugar. Caramelise the onions at medium heat for 4–5 minutes, stirring until golden brown.
Spread one of the bread slices with Kühne Horseradish with Cream and top with arugula, Kühne Dill Slices and roast beef. Spread the remaining Kühne Horseradish with Cream, top with caramelized onions and season with salt and pepper. Top sandwich with a second slice of bread and serve.