Mix mustard, sugar, vinegar and 3 tablespoons of oil. Wash the dill, dab dry, put something aside for garnishing. Finely chop the rest, stir into the sauce. Cut fillets into pieces, put them in a flat dish, pour sauce over them.
Peel onion and cut into rings. Spread over the maties. Wash the chives, dab dry and cut into rings. Peel, wash and roughly grate the potatoes. Mix in chives and season with salt and pepper.
Heat 1 tablespoon of oil and butter in a frying pan and bake 3-4 Rösti in portions. For each Rösti, add a large tablespoon of potato mixture to the pan and flatten (7-8 cm Ø). Fry for 2-3 minutes on each side until golden brown.
Keep warm, bake the rest of the Rösti in the same way. Serve with sour cream, matie, gravlax and prawns. Garnish with dill. Makes about 20 Rösti.