Preheat the oven to 200C/400F/Gas 6.
Remove any excess fat from the lamb meat and tie up with string to secure the meat.
Heat the oil in a pan over a medium heat. Add the lamb and fry for four minutes on each side, or until cooked to your liking. Remove from the heat and leave to rest for 3-4 minutes.
For the sauce, place the bones, thyme, bay leaf, beef stock and red wine into a saucepan over a high heat and bring to the boil. Boil to reduce the liquid volume by half, then add the cornflour and stir to thicken. Pour through a sieve and discard the solid ingredients. Pour the sauce into a serving jug.
For the potatoes, melt the butter in a large pan over a medium heat. When the butter is foaming, add the onions and gently fry for 2-3 minutes, until softened.
Place a deep chefs' ring onto a baking sheet. Place the onion into the chefs' ring and add the potato slices. Place into oven to bake for 10-12 minutes, or until golden and cooked through.
To serve, transfer the Lyonnaise potatoes onto a plate and remove the chefs' ring. Place the lamb alongside the potatoes and drizzle with the sauce.