Heat oven to 350 degrees. Sprinkle ribs with 1/2 tsp of the salt and all the pepper. In a 4 to 5 quart ovenproof pot, brown ribs on all sides in hot oil over medium high heat. Set ribs aside; discard all but 1 TB drippings from pot. Reduce heat to medium-low. Add carrots and garlic to pot; cook and stir about 10 minutes or until carrots are just tender.
Return the ribs to the pot. Add broth, wine, thyme, bay leaf and remaining 1/2 tsp of salt. Bring to boiling; remove from heat. Over pot; transfer to oven. Bake for 2 hours or until meat is very tender, adding frozen onions during the last 30 minutes of cooking.
Transfer ribs to a large, deep serving platter; cover and keep warm. Skim fat from cooking liquid; discard bay leaf and any bones that have fallen off ribs. If cooking liquid is too thin to serve as a sauce, bring to boiling and cook uncovered for 5 minutes to reduce slightly to about 2 1/2 cups. Whisk in mustard and horseradish. Pour sauce over ribs.
To make orange gremolata combine two tablespoons snipped fresh italian parsley; 2 cloves minced garlic and 2 tsp finely shredded orange peel. Top the ribs with the orange gremolata.