Pat the lamb dry with paper towels.
Combine garlic, herbs, olive oil, salt, pepper, lemon zest, and juice for the marinade.
Rub marinade over lamb, cover, and refrigerate for at least 8 hours.
Remove lamb from fridge 2 hours before cooking.
Preheat oven to 325°F (165°C).
Arrange onions, carrots, and celery in a roasting pan.
Place marinated lamb on vegetables and add water and chicken stock.
Cover tightly with foil and bake for 2.5-3 hours.
Increase oven temperature to 400°F (200°C), uncover, and brown for 15-20 minutes.
Let lamb rest for 15-20 minutes before slicing.
Slice and serve with sides of choice.