Peel potatoes and in a saucepan combine with salted water to cover. Bring water to a boil and simmer potatoes, covered, 10 minutes. Add carrots and simmer, covered 5 minutes. Drain vegetables and let cool. Quarter potatoes.
Preheat oven to 450 degrees. Pat rump roast dry and season with salt and pepper. Put meat in a roasting pan, fat side up, and scatter sliced onion around it. In another roasting pan toss pearl onions, garlic, carrots and potatoes with oil and season with salt and pepper. Roast vegetables in upper third of oven, stirring once, 20 minutes.
Put meat in lower third of oven with roast meat and vegetables, stirring vegetables for, 20 minutes. Reduce temperature to 300 degrees and roast meat and vegetables until a meat thermometer registers 130 degree for medium-rare meat and vegetables are golden, about 20 to 25 minutes. Transfer meat and vegetables to a platter and keep warm, covered loosley with foil.
Make gravy: add broth to meat roasting pan and deglaze pan over high heat, scraping up browned bits. In a small bowl, whisk together flour and water and add to simmering broth in a stream, whisking. Add vinegar and simmer, whisking, 3 minutes. Strain gravy through a fine sieve and reserve 1 cup for making deviled miroton.
Serve roast, sliced thin, with gravy, reserving narrower half of roast, unsliced, for making deviled miroton.
Recipes Courtesy of Gourmet Magazine