Peel potatoes, wash and boil in salted water for 15 minutes. Drain water, cool. Cut each potato in half lengthwise. Chop the onion. Mix broth, vinegar, 1 tbsp. l. oils and mustard. Season with salt, pepper and sugar to taste. Add potatoes and onions to the mixture, mix and leave for 1ºh. Salt and pepper the fish. Combine the remaining oil with soy sauce and 2 tbsp. l. lemon juice. Pour salmon with this marinade and leave for 1 hour, turning once. Wash the mint, tear off the leaves and chop finely. Mix with sour cream, season with salt, pepper, sugar and the remaining lemon juice, mix. Fry the fish on the grill or in a pan for 3-4 minutes. on each side, while pouring the marinade. Arrange on a plate with potato salad, drizzle with mint sauce, garnish with dill and serve immediately.