Take the lamb out of the refrigerator and leave in room temperature for about 45 minutes.
Meantime, make the rub by mixing the ingredients.
Cut the potatoes into bite-sized pieces, the pepper into chunks, the onion into thin wedges. Scatter these into a roasting dish and tuck the garlic cloves between them. Drizzle with some olive oil, then scatter some oregano on top.
Preheat the oven to 200°C.
Season the lamb whit salt and pepper and make a few slits on both sides. Put it on top of the veggies.
Drizzle with some oil then roast for 10 minutes to sear the lamb.
Remove the dish from the oven, then adjust the oven temperature to 165°C.
Insert the garlic slices in the slits, then cover the lamb with the rub using a silicon brush.
Add the water or wine to the roasting pan, then roast for about 2-2.5 hours. Until the lamb starts falling off the bone. You can cover the dish with foil for the first hour, if preferred.
Serve warm, with fresh parsley scattered on to.