1. Place garlic, chopped rosemary, thyme leaves, orange juice, wine, and 3 tablespoons olive oil in the jar of a blender; pulse 5 or 6 times to combine. Generously season lamb with salt and pepper and transfer to a large resealable plastic bag; add liquid from blender and seal bag, squeezing out all the air. Transfer to refrigerator and let marinate for at least 3 hours and up to 24 hours.

2. Remove lamb from marinade and wipe off any excess with a paper towel. Let stand at room temperature for 1 hour.

3. Preheat oven to 425 degrees.

4. Place onion, carrots, fennel, and leeks in a large bowl. Add 2 tablespoons olive oil and season with salt and pepper; toss to combine and transfer to a large roasting pan. Top with remaining 3 sprigs of rosemary and thyme. Place lamb on top of vegetables in roasting pan; drizzle with olive oil and season with salt and pepper.

5. Transfer roasting pan to oven and roast for 10 minutes. Reduce heat to 350 degrees and continue cooking until lamb reaches 125 to 130 degrees on a meat thermometer for medium-rare, about 1 hour more. Remove from oven and let stand for 20 minutes before slicing against the grain. Serve with vegetables.