Wash the lamb under cold running water, pat dry and pour all over half of the lemon juice. Place in the refrigerator for at least 4 hour or better overnight.
1 1/2 hours prior to cooking take the lamb out of the refrigerator to come to room temperature.
15 minutes prior to cooking preheat the oven at 450F, rack set at the middle.
Season the lamb chunks with the salt and pepper and place them in a very large roasting pan. Add into it the potatoes, onions, garlic, bay leaves and all-spice, toss and pour on them the olive oil. Cover and place in the oven.
After an hour, reduce the oven temperature to 325F and cook for further 2 hours. After 2 hours remove from the oven, uncover and pour the lemon juice on the meat and potatoes.
Return to the oven 375F and cook for 25-30 more minutes and until nicely browned. Enjoy.