Honey-Vinegar Leg of Lamb with Fennel and Carrots

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.