Preheat the Oven: Preheat your oven to 375°F (190°C) to prepare for baking.

Sauté the Onions: In a large skillet, heat the vegetable oil over medium-high heat. Add the chopped onions and sauté until they become soft and golden brown. Remove the onions from the skillet and set them aside.

Brown the Lamb: Using the same skillet, add the cubed lamb pieces. Cook them in batches if necessary, ensuring each piece is browned evenly on all sides. This process should take about 12 to 15 minutes. Once done, drain any excess fat.

Assemble the Hotpot: In a 9×13 inch baking dish, layer half of the thinly sliced potatoes on the bottom. Season with salt and pepper. Spread the sautéed onions over the potatoes, followed by the browned lamb. Sprinkle with chopped fresh thyme and additional seasoning as desired. Top with the remaining potato slices, arranging them neatly.

Add Stock and Butter: Pour the chicken or lamb stock over the assembled layers. Drizzle the melted butter over the top layer of potatoes to encourage browning.

Bake the Hotpot: Cover the baking dish with a lid or aluminum foil and place it in the preheated oven. Bake for 1½ to 2 hours, removing the cover during the last 30 minutes to allow the top layer of potatoes to crisp and turn golden brown. If the dish appears to be drying out, add more stock as needed.

Serve: Once cooked, remove the hotpot from the oven and let it rest for a few minutes before serving. This allows the flavors to meld together beautifully.