Scrub potatoes, beets and carrots and tightly wrap in foil severally; bake in a preheated oven at 180°C for 40 minutes. Cool, peel and cut into small cubes. Put all the ingredients for the sauce in a saucepan, start heating and stir until the sauce thickens. Remove from fire just before boiling. Season with salt and pepper. You can reduce but not increase the amount of sugar. Peel cucumbers, clean off seeds and cut into cubes. Cut the herring into small cubes, mix with cucumbers and vegetables in a bowl. Dress the vinegret with mustard sauce.