In a bowl, combine minced garlic, chopped rosemary, olive oil, salt, pepper, and lemon juice to create the marinade.

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

Preheat your oven to 350°F (175°C).

In a roasting pan, place the carrots, potatoes, and onion. Drizzle with olive oil and season with salt and pepper.

Place the marinated lamb leg on top of the vegetables in the roasting pan. Roast in the preheated oven for about 1.5 to 2 hours, or until the internal temperature reaches 145°F (63°C) for medium-rare.

Remove the lamb from the oven and let it rest for 15 minutes before slicing. Serve with the roasted vegetables.