Place herring in a bowl and cover with salt and let stand for two days, turning occasionally.
Remove dorsal fin and cut along dorsal fin with a knife carefully cutting fish from backbone on both sides. Remove bones. For the marinade, mix 200 ml (approximately 7 ounces) very cold water, 2 ml (approximately 1/2 teaspoon) oil, dill and season with a pinch of salt. Place herring in a single layer in pan or bowl and sprinkle with marinade. Place a thin layer of onion rings over herring. Repeat steps until ingredients are used. Cover and refrigerate for about 2 days and store for up to 8 days in refrigerator.