Ask your butcher to remove the rind and half the thickness of fat from a loin of pork.

Remove the pork from the refrigerator 4 hours before you plan to cook it. Score the fat on the loin with a sharp knife in a crisscross pattern. Cut each garlic clove into 4 slices. Pierce the pork loin in 12 places with a thick skewer and insert a garlic slice and a little chopped onion mixture into each hole.

Mix the softened butter, crumbled thyme, bay leaf and mustard to a smooth paste and rub this well into the pork. Sprinkle with salt and pepper, and let the meat stand at room temperature for 4 hours.

Heat the oven to 230°C (450°F) gas 8. Arrange the meat, fat side up, in a roasting tin and brown it for 15 minutes. Reduce the temperature to 180°C (350°F) gas 4 and continue to roast until the meat is done, about 1 1/4 - 1 1/2 hours.

Remove as much fat as possible from the juices in the tin, blotting them with a strip of absorbent paper. Mash together the flour with the butter and add this a little at a time to the pan drippings. Stir constantly over a low heat until the sauce thickens. Pour this mustard butter into a small serving dish.

Garnish the pork with sprigs of watercress and serve with pureed potatoes and the mustard butter.