Preheat the oven to 170C/150C fan/Gas 3.
Season the lamb chops with plenty of salt and freshly ground black pepper. Heat the oil in a frying pan over a medium heat, then fry the lamb chops for 2-3 minutes on each side, or until browned all over. Transfer the lamb chops to a medium casserole.
Fry the carrot, celery and onion in the same frying pan for 4-5 minutes, or until coloured and softened.
Stir the flour into the vegetables and continue to fry for 2-3 minutes, or until the flour turns a biscuit colour.
Pour in the beef stock and stir well until the liquid has thickened and there are no visible lumps of flour.
Transfer the vegetables and thickened stock to the casserole. Add 7 thyme sprigs and the bay leaf, then bring the mixture to the boil. Reduce the heat until the mixture is just simmering.
Melt the butter in a small saucepan over a medium heat.
Arrange the potato slices on top of the hotpot, overlapping the edges slightly. Brush the potato slices all over with the melted butter. Sprinkle the top with a few thyme leaves from the remaining sprig of thyme.
Bake the hotpot in the oven for 1-1½ hours, or until the potato topping is crisp and golden-brown and the vegetables are tender.