Heat a heavy bottomed large, wide pot or pan over high heat. Cut the sirloin into large cubes. Add oil to the pan and heat. Add meat and caramelize for 2 minutes on each side; remove from pan to a shallow dish and cover loosely with foil. Reduce heat to medium high. Add bacon to the pan and brown bacon at edges, rendering fat. Add onions and celery to the pan, bay and thyme. Stir and cook vegetables 5 minutes. Add flour to the pan, stir and cook another minute. Whisk in wine and mustard and scrape up pan drippings. Add stock. Cover and bring liquid to a boil, 1 to 2 minutes.
Mix watercress with biscuit mix and add beef stock, (instead of water), according to directions on box. Add meat back to the pan and settle it in so that it is covered with sauce. Season with salt and pepper, to taste. Drop large spoonfuls of watercress biscuit mix into the pan onto the surface of the stew and cover the pan. Cook 8 minutes until dumplings plump. Serve stew with dumplings.