1. Rinse pork and pat dry. Combine juice, garlic, and rosemary in a large resealable plastic bag. Add pork, season with salt and pepper, then seal bag, pressing out any air. Turn until pork is coated well. Marinate, chilled, turning bag occasionally, 1 hour, placing bag in a large bowl to catch any leaks.
2. Meanwhile, preheat oven to 425 degree F and arrange rack in center. Toss together fennel, onion, and oil on a large nonstick baking pan; season with salt and pepper.
3. Roast, stirring occasionally, until just tender and edges are golden, about 30 minutes. Remove pan from oven and push vegetables to one side, leaving just enough room for pork on the other side. Remove pork from marinade and transfer to pan. Spoon some garlic and rosemary from marinade over pork (reserve marinade).
4. Roast, stirring vegetables occasionally, until they are golden brown and an instant-read thermometer inserted in center of pork by 2 inches registers 155 degree F, about 30 minutes.
5. Transfer vegetables to one side of a serving platter and keep warm, covered. Transfer pork in pan to rack and let stand 10 minutes before carving into 1/4-inch-thick slices.
6. Meanwhile, bring reserved marinade to a boil in a small saucepan. Boil until reduced by half, about 4 minutes, for sauce.
7. Arrange pork slices next to vegetables on platter and spoon sauce over top. Makes 6 servings.