In a small saucepan melt the butter over a moderate heat, add the onion, garlic, thyme and peppercorns and cook for around 5 mins until the garlic and onions are soft.
Add the white wine and turn up the heat – keep cooking this until almost all the liquid is evaporated. Next add the Worchester sauce and again reduce until almost no liquid remains.
Turn theheat back down to medium and add the flour, stir to form a paste. Add the stock in small quantities stirring all the time, until you reach a nice sauce consistency that coats the back of the spoon, add a little more stock and allow the sauce to cook, very slowly, for around two minutes.
Season and if you like, finish with a little finely chopped parsley and another knob of butter.