Heat a large frying pan over medium-high heat and brown the lamb chops on both sides. You are not trying to cook them through at this point, just adding colour and flavour.
If the chops are particularly fatty, you may not need any extra oil in the pan.
Remove the lamb and set aside.
Add the onions, leek and carrot to the same pan and cook gently until beginning to soften. Add the garlic and cook for another minute.
If you want a slightly thicker gravy, stir in a tablespoon or two of flour and cook briefly before adding the stock.
Pour in the stock and Worcestershire sauce, then add the mustard and thyme if using. Stir well and season with black pepper.
Arrange the lamb in a deep casserole dish and spoon the vegetables and stock around it. The liquid should come partway up the meat rather than completely covering it.
Peel the potatoes if you prefer, then slice them thinly.
Arrange the potato slices over the top in overlapping layers, rather like roof tiles.
Brush or dot the potatoes with a little butter or olive oil and season lightly.
Cover the casserole and cook at 160–170°C fan-forced for around 4½ hours, or until the lamb is tender.
Remove the lid for the final 30–40 minutes.
If the potatoes need more colour, increase the oven to around 200°C for the last 15–20 minutes, watching carefully so they become golden rather than burnt.
The exact cooking time will depend on how thick your chops are, so use tenderness rather than the clock as your guide.