Preheat the oven to 170°C (150°C fan), Gas Mark 3½.
Heat a large flameproof casserole over a medium heat. When the pan is hot, add the cumin seeds and cardamom pods and dry-toast for about 2 minutes, stirring occasionally to prevent burning, until the spices release their aroma.
Pour enough oil into the pan to coat the base. When the oil is hot, add the onions and fry for 6–8 minutes, or until translucent and the edges start to brown. Add the meat and cook for a few minutes, stirring to coat in the oil and cumin seeds. Add the garlic cloves, shallots and carrots and stir for a couple of minutes, then add the ground cumin, ginger, cinnamon, cayenne pepper, mustard powder and flour and a generous seasoning of salt and pepper. Mix well, then stir in the stock.
Arrange the potato slices on top of the meat, slightly overlapping them. Brush with the melted butter, sprinkle with thyme leaves and season again with salt and pepper, then cover the casserole with a lid and transfer to the oven to cook for 1½ hours.
Take the hotpot out of the oven and remove the lid. Increase the oven temperature to 220°C (200°C fan), Gas Mark 7.
Return the casserole to the oven, uncovered, and cook for a further 30–40 minutes, or until the potatoes are golden brown.
Serve immediately.