Preheat your oven to 325°F (160°C).
In a large, deep baking dish or casserole, arrange the ingredients in the following order:
- A layer of sliced potatoes, overlapping slightly
- A layer of sliced onions
- A layer of diced lamb
- Season with salt, pepper, and a few sprigs of thyme
Repeat the layers of potatoes, onions, and lamb, seasoning each layer.
Pour the beef or lamb stock over the layers, ensuring the ingredients are just barely submerged.
Top the casserole with a final layer of sliced potatoes, overlapping them slightly.
Cover the dish with a lid or aluminum foil and bake for 2-2.5 hours, or until the potatoes are tender, and the lamb is fall-off-the-bone tender.
Remove the lid or foil during the last 30 minutes of cooking to allow the top layer of potatoes to crisp up.
Allow the lamb hotpot to rest for 10-15 minutes before serving.