Take lamb out of the fridge 1 hour before you are ready to work with it.
Preheat oven to 450˚F.
In a food processor or blender, combine worcestershire sauce, apple cider vinegar, sugar, soy sauce, garlic, sun dried tomatoes, mint leaves, parsley, lemon juice, and hot water; pulse to make a chunky sauce. Set aside.
Lay lamb shoulder, fatty-side up, on a working surface, like a large cutting board.
Using a sharp knife, make several slashes across the top.
Rub the lamb with the olive oil; season with salt and pepper.
Coat lamb with HALF of the prepared sauce. Keep the other HALF in the fridge until ready to serve the lamb.
Arrange onion rings and smashed garlic in the base of a roasting pan.
Place the lamb shoulder on top of the onions, fatty side up.
Pour 1 cup of water in the roasting pan.
Cover with aluminum foil.
Put roasting pan in the oven and turn down the heat to 350˚F.
Roast for 3 hours.
Remove foil and increase heat to 425˚F. Check to see if there’s any liquid in the pan; if not, add a cup of water.
Continue to roast for about 20 to 25 minutes, or until browned on top and internal temperature registers at 145˚F. Start checking internal temperature at around the 15-minute mark.
When done, remove from oven.
Transfer lamb to a cutting board and loosely cover with foil; let it rest for 20 minutes.
Drizzle with reserved sauce.