The best gravy is made with the juices from the meat and that is exactly what you get with this recipe. Add to that the beef stock, fresh thyme, shallots and garlic and you have the most incredible base for a gravy. Strain all of the liquid from the slow cooker and put it in to a saucepan. Stir in a cornflour slurry (1 tbsp cornflour mixed with a little of the liquid) and cook on low for 12-15 minutes until thickened.