Preheat the oven to 200°C (Gas Mark 7). To make the stuffing, place a frying pan over a medium heat and melt the butter, then add the onion and cook, stirring, until soft (8-10 minutes). Crush in the garlic clove and add the bacon pieces, then cook for a few minutes more until the bacon is cooked through and just turning crispy. Take the pan off the heat and leave to cool. Put the breadcrumbs, cheese, herbs and lemon zest in a large mixing bowl and stir to combine, then add the egg and mustard and the contents of the bacon pan. Season to taste with salt and pepper, then stir again until evenly mixed. Set aside.

 Next, carefully remove the string from the Beef Sirloin (see note in Ingredients), and have a roasting tray ready to hand. Make a length-wise cut in the top of the meat, down to about two-thirds of the way through it; this will be the ‘pocket’ that holds the stuffing. Spoon the stuffing into the pocket, pressing gently down to fill it completely, then re-use the butchers’ string (or just use kitchen string) to tie the sirloin up again at roughly 2cm intervals, so that the stuffing is secure. Transfer the joint to the roasting tray and rub the olive oil all over the meat, then season with salt and pepper. Put the sirloin into the preheated oven and roast for 20 minutes, then reduce the oven temperature to 180°C (Gas Mark 6) and continue cooking for 25-30 minutes (now is a good time to prepare any accompanying veg). When cooked through, remove the beef from the oven and carefully spoon or pour out about 2 tbsp of the roasting juices from the tray, then set the meat aside to rest while you make the gravy.

Put the roasting juices in a saucepan over a medium heat and add the flour, then cook, stirring, until the mixture bubbles and thickens. Add the wine and stock a little at a time, stirring as you go, until the mixture has thickened slightly again and is piping hot. Remove the string from the beef and carve the meat into slices, then serve with your choice of veg and the hot gravy alongside.