Heat oven to 200C/fan 180C/gas 6. Set the leg of lamb into a roasting tin. Season with a little salt and plenty of black pepper. In a bowl, mix together the mustard and plain flour to a smooth paste. Spread it all over the lamb. Roast for 20 mins, then reduce the heat to 160C/fan 140C/gas 3 and cook for a further 1 hr 20 mins. Remove the lamb from the oven and leave to stand, covered in foil, for 15 mins.
For the sauce, mix together 2 tbsp cold water and the cornflour to make a smooth paste. Beat together the egg yolks and lemon juice in a small saucepan and add 2-3 tbsp of the hot stock, whisking well. Stir in the cornflour paste and heat gently. Pour over the rest of the stock, then over a low heat stirring constantly, cook for 7-10 mins until the sauce has thickened. Do not boil. Add the spring onions, adjust the seasoning to taste, and serve hot with thin slices of the lamb.