Preheat oven to 180°C Using a small knife, cut 1cm slits all over lamb. Stuff each incision with a slice of garlic. Combine lemon juice, 1/4 cup oil, rosemary, fresh and dried oregano, 2 tsp paprika, 1 tsp sea salt and 1/4 tsp freshly ground black pepper in a small bowl. Place lamb in a roasting pan, pour mixture over then turn lamb to coat evenly. Cover lamb with foil and roast for 1 hour.
Meanwhile, place potatoes in a large saucepan of cold water. Bring to the boil and cook over high heat for 4 minutes. Drain. Return potatoes to pan with remaining oil, paprika, lemon rid and 2 tsp sea salt. Toss to combine, set aside.
Remove foil from lamb. Add potatoes to pan. Stir to coat in cooking juices then return to oven and cook, uncovered for a further hour, turning potatoes occasionally. Remove pan from oven and cover loosely with foil. Stand for 15 minutes.
Place potatoes on a platter. Thinly slice lamb and place on top. Sprinkle feta over lamb and spoon pan juices over. Serve.