Heat the oven to 190°C/gas 5. Place the potatoes, red pepper and tomato in a large, flameproof oven dish or tin and season with the salt and pepper. Put the leg of lamb on top and pour over the lemon juice and olive oil, then add the garlic halves to the dish. Sprinkle all over with the oregano .Pour the water into the dish and cover with foil, sealing the edges well to retain the moisture as it cooks. (A large enamelled, lidded, cast-iron pot,such as Le Creuset, would also be suitable to cook this in.)
Cook in the oven for 2½–2¾ hours until the meat is falling off the bone, adding a little more water, if necessary, halfway through to keep the vegetables moist. Remove the foil (or lid) and add the feta, then return to the oven for 10 more minutes. After this time, check the consistency of the juices. If the dish seems a little dry, add a little more water. If too dilute, transfer the meat and vegetables to a serving dish and reduce down the liquid on the hob.
Serve the meat in chunks or thick slices with the vegetables and juices.