Beef with walnuts

3 pounds beef stew meat 1 1/2 pounds yellow onions  finely chopped 1/4 cup Olive Oil   For the Stock  2 ounces Sour Plums , dry (also called golden prunes or[...]

4 x Fillets steak, each weighting about 140g     Salt and freshly grnd black pepper 3 Tbsp. Vegetable oil 4 lrg Flat mushrooms 20 gm Unsalted butter 4 Tbsp.[...]