Preheat your oven to 325°F (160°C) and arrange the thickly sliced onions in the bottom of a large roasting pan to create a bed for the meat.

In a medium bowl, whisk together the Greek yogurt, tomato paste, minced garlic, olive oil, oregano, cumin, paprika, lemon juice, salt, and black pepper until smooth.

Place the lamb shoulder on a cutting board and use a sharp knife to score the fat cap in a crosshatch pattern, cutting about 1/4 inch deep.

Rub the yogurt marinade generously all over the lamb, ensuring it gets into the scored cuts and covers the underside of the meat.

Place the marinated lamb on top of the onions in the roasting pan and carefully pour the water into the bottom of the pan, avoiding the meat so the marinade stays intact.

Cover the roasting pan tightly with heavy-duty aluminum foil to seal in the steam and moisture.

Roast in the preheated oven for 3.5 to 4 hours, or until the meat is extremely tender and pulls away easily from the bone with a fork.

Remove the foil and increase the oven temperature to 400°F (200°C).

Roast uncovered for an additional 15 to 20 minutes until the crust is golden brown and slightly charred in spots.

Remove the lamb from the oven and let it rest for at least 15 minutes before shredding or slicing.

Serve the tender lamb warm over a bed of rice pilaf or with flatbread, spooning some of the pan juices and onions over the top.