Step 1 Preheat oven to 240ºC (220ºC fan-forced). Heat a heavy-based roasting pan in the oven for 10 minutes.
Step 2 Meanwhile, combine spinach, feta, onion, garlic and oregano in a large bowl. Season with freshly ground black pepper.
Step 3 Place leg of lamb, fat-side down, on a clean work surface. Open to sit flat. Spoon spinach mixture into the centre, then roll up to enclose. Secure roll of lamb with white unwaxed kitchen string tied at 2cm intervals.
Step 4 Brush lamb with oil. Remove pan from oven and place over stove on medium heat. Add lamb and cook for 3 minutes. Turn and cook for a further 3 minutes or until browned all over. Reduce oven temperature to 200ºC (180ºC fan-forced). Return lamb to oven and roast for 1 1/2 hours for medium done, or until cooked to your liking.
Step 5 Remove from oven. Cover with foil and set aside for 10 minutes to rest. Carve and serve with lemon.