Preheat the oven to 375°F (190°C).

In a bowl, mix the softened butter, minced garlic, rosemary, thyme, salt, and pepper.

Rub the garlic butter mixture all over the leg of lamb, ensuring it is well coated.

Place the lamb in a roasting pan and surround it with carrots, potatoes, and onion.

Roast in the preheated oven for about 90 minutes, or until the internal temperature reaches 135°F (57°C) for medium-rare.

Baste the lamb with the pan juices every 30 minutes.

Remove from the oven and let it rest for 15 minutes before slicing.