I struggle to write up the recipe for anything I've cooked, it feels overly prescriptive when I have made the dish to suit my tastes entirely. As I feel this is a recipe that you can play around with, and be fairly liberal with the quantities, I'm ditching the standard recipe format for something altogether more relaxed.
Peel and finely slice some onions- four will do for about 5 people. In a heavy bottomed saucepan, melt a generous amount of unsalted butter over a low heat. Add the onions and cook very slowly until golden and collapsing. Turn the heat up, and stir in a tablespoon of plain flour. Let the onions caramelise, stirring all the time so they do not not burn. Now add about 750 ml stock- light chicken or vegetable are good, but if you can get hold of some porcini mushroom stockcubes from an Italian deli or elsewhere, use these. They add a lovely depth to the gravy Bring to a simmer and add some Marsala or Madeira wine, about 125ml. Cook until the harsh alcohol smell evaporates, and season with some Worcestershire Sauce, and a touch of Tabasco if you like a little heat. Let it simmer until you get a consistency you like, and add salt and freshly ground pepper to taste.