In a 6 quart Dutch oven on medium heat, add short ribs a few at a time, and brown ribs on all sides, about 45 min.
Peel onions. Scrub potatoes well, pare a 1” strip of skin, from center of potatoes. Pare carrots.
Pour off drippings, reserving 2 tablespoon in Dutch oven. Saute chopped and whole small onions in reserved drippings in pan for about 10 min, stirring occasionally. Remove whole onions, set aside. Add garlic, peppercorns, bay leaf, Worcestershire sauce, marjoram, beef broth, and 1/2 cup water. Add browned short ribs. Cover pan and bring to a boil. Reduce heat and simmer, covered for 1 hour, turning meat to coat.
After 1 hour, add carrots, browned small onions, and potatoes. Simmer covered for 45 min.
Meanwhile, In a small bowl with a electric mixer, beat cream until stiff. Stir in horseradish, mustard and a pinch of salt. Cover and refrigerate.
Test meat and vegetables for doneness, if necessary, cook a bit more until everything is fork tender. Arrange beef and vegetables on a platter.
Skim fat from pan juices. Mix flour with ¼ cup water until smooth. Stir into pan juices, bringing to boil over medium high heat. Reduce heat and simmer for 3 min. Pour through strainer over short ribs. Serve with Horseradish cream on the side.