Preheat the oven to 170°C.
Season lamb chops with salt and pepper and brown in oil in a frying pan.
Transfer browned lamb to a casserole dish.
Fry carrot, celery, and onion in the pan until softened.
Stir flour into vegetables and cook for 3 minutes until light brown.
Pour in beef stock and stir until the sauce thickens.
Add vegetables, stock, 7 thyme sprigs, and bay leaf to the casserole and simmer.
Melt butter in a small saucepan.
Layer potatoes over the lamb, brush with melted butter, and sprinkle with remaining thyme.
Bake in the oven for up to 90 minutes until potatoes are crisp.