Position rack in center of oven; preheat to 450°F. Using vegetable peeler, remove peel (orange part only) in strips from 3 blood oranges. Place pork in large roasting pan. Rub with 2 tablespoons olive oil; sprinkle with salt and pepper. Toss onion, orange peel, and rosemary leaves with remaining 1 tablespoon oil in medium bowl; sprinkle with salt and pepper. Arrange onion mixture around pork. Roast until pork and onion are beginning to brown, about 25 minutes.
Meanwhile, squeeze enough juice from oranges to measure 3/4 cup; pour juice into small saucepan. Add wine, broth, and garlic. Boil until reduced to 1 1/4 cups, about 7 minutes.
Add 1/2 cup orange juice mixture to roasting pan. Baste pork with pan juices. Reduce oven temperature to 350°F; continue roasting pork until thermometer inserted into thickest part of pork registers 150°F, basting often and adding more orange juice mixture as needed, about 30 minutes longer.
Place pork on platter and cover loosely with foil. Place roasting pan over medium-high heat; add remaining 3/4 cup orange juice mixture to pan. Boil until slightly thickened, about 3 minutes. Remove strings from roast. Slice pork; place on platter. Pour sauce and onion mixture over and around pork. Garnish with rosemary sprigs and orange slices.
Adapted from Bon Appetit 2007