Preheat oven to 500 degrees F (260 degrees C). Grease a roasting pan with olive oil.
Rub seasoned salt over leg of lamb.
Mix flour and mustard together in a bowl. Rub over leg of lamb. Transfer to the prepared roasting pan.
Roast leg of lamb in the preheated oven until browned, about 20 minutes.
Reduce oven temperature to 325 degrees F (160 degrees C). Add orange juice, onion, and water to the roasting pan. Continue baking lamb, basting with orange juice and water every 30 minutes, until tender, about 2 1/2 hours.