Place beef ribs in a large resealable plastic bag. Add onions, tomato sauce, water, vinegar, sugar, parsley, horseradish, mustard, salt, and black pepper. Squeeze excess air out of bag and seal. Marinate in refrigerator overnight or up to 24 hours, turning bag several times.
Transfer beef and marinade to a large pot and bring to a boil. Reduce heat to medium-low, cover, and simmer until beef is tender and falling off the bones, about 2 hours. Remove beef from liquid with a slotted spoon.
Preheat an outdoor grill for medium-high heat and lightly oil the grate.
Arrange beef on preheated grill and cook until browned on all sides, about 10 minutes.
