Allow leg of lamb to rest at room temperature for 1 hour before cooking.

Preheat the oven to 400 degrees F (200 degrees C). Place the oven rack in the lower third of the oven.

Whisk together 1/2 cup olive oil, vinegar, honey, garlic, rosemary, black pepper, red pepper flakes, and bay leaf in a small bowl until well combined. Rub all over lamb, then season with 2 tablespoons coarse salt. Set aside.

Place carrots, turnips, potatoes, beet, and yam in a large roasting pan. Toss with 2 tablespoons olive oil and a pinch of salt until evenly coated. Place lamb on top of vegetables.

Roast in the preheated oven for 20 minutes. Reduce oven temperature to 325 degrees F (165 degrees C) and continue roasting to desired degree of doneness, about 90 minutes more for medium-rare. An instant-read thermometer inserted into the center of lamb should read 130 degrees F (54 degrees C).

Transfer lamb to a cutting board and loosely tent with aluminum foil for 30 minutes before carving. Place vegetables on a serving platter and cover with aluminum foil to keep warm until ready to serve.