Preheat oven to 450°. Finely shred zest from 2 large clementines; save clementines for another use. In a food processor, whirl zest, chile flakes, ginger, garlic, and salt to mince. Blend in oil. Put meat in a 12- by 17-in. roasting pan, rub all over with chile mixture, and set fat side up.
Roast beef until browned, 20 to 25 minutes. Reduce heat to 325° and cook until an instant-read thermometer inserted in thickest part of meat reaches 90°, about 20 minutes.
Juice remaining clementines (to yield about 2 cups). Bring a large saucepan of water to a boil. Add clementine peels and boil gently 5 minutes, stirring occasionally. Drain; rinse with cool water. Scrape out and discard pulp and white pith. Cut peels in half, then stack a few at a time and cut into long, thin strips. In a bowl, combine clementine strips, sugar, soy sauce, sherry, and 2/3 cup clementine juice.
When meat is at 90°, pour clementine soy mixture around it. Cook, stirring sauce occasionally, until meat reaches 130° for medium-rare, 30 to 40 minutes; as pan juices begin to caramelize, stir in a few tbsp. clementine juice if needed to prevent scorching.
Transfer beef to a cutting board and let rest, tented loosely with foil, 15 minutes. Pour remaining clementine juice into roasting pan, set pan over high heat, and cook, stirring often, until juices are thickened and shiny, about 7 minutes. Scrape sauce into a bowl.
Snip twine from roast. Thinly slice meat crosswise. Serve with sauce and onions.
*Ask butcher to tie the roast, or do it yourself.
Note: Nutritional analysis is per serving.