- 2 garlic cloves, chopped
- 1 cup fresh flat-leaf parsley leaves
- 1 tablespoon fennel seeds, crushed
- 1/2 cup fennel fronds plus more for serving
- 1/2 cup olive oil, divided
- Kosher salt, freshly ground pepper
- 1 7–9 pound bone-in leg of lamb, tied
- 1/2 cup red wine vinegar
- 1/3 cup honey
- 4 fennel bulbs, sliced 1/2″ thick
- 16 small carrots (about 2 pounds), unpeeled, halved lengthwise if large
- Pulse garlic, parsley, fennel seeds, and 1/2 cup fennel fronds in a food processor until very finely chopped. With motor running, slowly add 1/4 cup oil and process until smooth; season generously with salt and pepper (mixture should taste quite salty as this is the only seasoning meat will get). Rub parsley mixture all over lamb. Transfer to a roasting pan, cover, and chill at least 8 hours.
- Let lamb sit at room temperature 1 hour. Bring vinegar and honey to a boil in a small saucepan. Reduce heat and simmer, stirring occasionally, until slightly thickened, 8–10 minutes. Set glaze aside.
- Place racks in lower third and middle of oven and preheat to 325°F. Toss fennel and carrots with remaining 1/4 cup oil in a medium bowl and arrange half around lamb in roasting pan; place remaining vegetables on a rimmed baking sheet.
- Roast vegetables on baking sheet on lower rack and lamb on middle rack until an instant-read thermometer inserted into the thickest part of lamb registers 100°F, about 1 hour.
- Increase oven to 450°F, brush roast with glaze, and continue to roast until an instant-read thermometer registers 120°F, 20–25 minutes longer. Transfer lamb to a cutting board and let rest 30 minutes before carving (temperature will rise to 140°F for medium-rare while lamb is resting).
- Continue to roast vegetables, tossing occasionally, until golden and tender, 20–25 minutes longer.
- Serve lamb with vegetables, topped with fennel fronds.
- DO AHEAD: Lamb can be rubbed with parsley mixture and vegetables can be prepped 1 day ahead. Cover separately and chill.