Rinse beef tongue and place in a large pot. Cover with water and add the salt and chopped onion. Bring to a boil, reduce heat and simmer for 2 to 4 hours or until tongue is tender.
Remove tongue from cooking liquid and set aside until cool enough to handle.
Cut through tough outer skin and peel it off. Trim any fat or gristle from base of tongue. Slice tongue crosswise into 1/4 inch thick slices.
Slice loaf of rye bread in half lengthwise. Scoop out a small amount of the soft interior.
Spread mustard over each half.
Lay onions and tongue slices over bottom half of bread and top with other half and slice into individual sandwiches.
