Place rabbit in 1-gallon resealable baggie or large baking dish. Combine honey mustard, ½ cup olive oil, 2 sprigs rosemary, onion, and juice of ½ a lemon in medium bowl. Season with salt and pepper. Rub marinade into meat of rabbit, inside and out. Marinate overnight in refrigerator.
Heat oven to 350°. Place leeks, carrots, and potatoes in large roasting pan. Toss with salt and pepper, remaining 8 sprigs rosemary, and remaining ¼ cup olive oil. Spread vegetables in bottom of roasting pan, making a bed for the rabbit. Place rabbit on top of vegetables positioned on its side. Squeeze juice of remaining ½ a lemon on top of the whole dish.
Roast for 1 hour. Flip rabbit over and continue roasting for an additional hour. Transfer rabbit and vegetables to a platter and cover with aluminum foil; let rest for 15 minutes. Slice rabbit and serve with roasted vegetables, ladling juice on top of each serving.