In a heavy-based frying pan heat 25ml vegetable oil, add the beef trimmings and cook over a medium heat until evenly brown. Drain the beef trimmings into a colander. Return the pan to the heat and add the remaining oil to it. Add the shallots and mushrooms and sauté until golden brown and then drain into the same colander with the beef trimmings. Place the beef trimmings, mushrooms, shallots, veal stock and thyme into a heavy-based saucepan and bring to the boil. Reduce the heat to a simmer and cook for 1½ hours, skimming the surface frequently to remove any impurities that float to the top. Pass the sauce through a sieve lined with a double-layer of muslin cloth into a clean pan and over a medium heat, reduce to 1½ litres. Skim the surface frequently and season to taste with salt and freshly ground pepper. Once again, pass the stock through a fine sieve lined with muslin cloth into a clean stainless steel container. Refrigerate to solidify any fat that may rise to the surface. Remove the fat before using the veal jus.