In a bowl, mix olive oil, garlic, rosemary, thyme, oregano, lemon juice, salt, and pepper to create the marinade.

Rub the marinade all over the leg of lamb, ensuring it's well coated. Cover and refrigerate for at least 2 hours, preferably overnight.

Preheat the oven to 325°F (160°C).

Take the lamb out of the refrigerator and let it sit at room temperature for about 30 minutes before roasting.

Place the carrots, potatoes, and onions in the roasting pan and drizzle with olive oil, salt, and pepper.

Place the marinated lamb on top of the vegetables in the roasting pan.

Roast in the preheated oven for about 2 hours, or until the internal temperature reaches 145°F (63°C) for medium-rare.

Baste the lamb with the pan juices every 30 minutes.

Once cooked, remove the lamb from the oven and cover it loosely with aluminum foil. Let it rest for 15 minutes before carving.

Serve the lamb with the roasted vegetables and enjoy!