Peel, wash and thinly slice the potatoes. Grease a gratin dish. Layer the potatoes. Season with salt and pepper. Mix cream and mustard and pour over the potatoes.
Cook in the preheated oven (electric cooker: 200 °C/ gas: level 3) for 45-50 minutes. Wash fish, pat dry and cut into large pieces. Sprinkle with lemon juice. Season with salt and pepper.
Clean the tomatoes, wash and pat dry. Put fish, tomatoes and parsley alternately on 8 wooden skewers. Heat oil in a pan. Fry the fish skewers for 8-10 minutes while turning. Serve potato gratin with the fish skewers.
Garnish with dill and lemon.