Heat the vegetable oil in a large frying pan over medium-high heat. Add the lamb and cook until browned on all sides, about 5 minutes. Season with saffron, salt and pepper to taste; stir in all but 4 tablespoons of the onion, and pour in the water. Bring to the boil, then cover, reduce heat to medium-low, and simmer until the lamb is tender, about 15 minutes. Uncover the pan, stir in the butter and allow the sauce reduce 5 to 10 minutes to desired consistency. Season to taste with salt and pepper, then pour into a serving dish. Sprinkle with the remaining chopped onions and parsley. Garnish with lemon wedges to serve.