Preheat the oven to 170°C.
Rub the lamb with olive oil and season with salt and pepper.
Grind coriander seeds, fenugreek, mustard seeds, cumin, fennel, peppercorns, cinnamon, and cloves into a powder.
Stir ground turmeric into the spice powder.
Blend chilli, garlic, and ginger into a paste using a small amount of water if needed.
Brown the lamb in a casserole pot with vegetable oil then remove and set aside.
Fry sliced onion in the same pot until soft and slightly colored.
Add 37.5 ml of curry powder, the prepared paste, curry leaves, cardamom, and bay leaves to the pot.
Return lamb to the pot and add tomatoes, stock, and tamarind.
Roast covered for 3 to 4 hours at 170°C until tender, basting occasionally.
Remove the lamb and simmer the sauce until it reduces and thickens.
Shred the lamb with a fork and return it to the thickened sauce.
Boil 2cm potato cubes in water for 5 to 8 minutes then drain.
Fry mustard seeds in oil until they pop, then add chilli powder, turmeric, and potatoes.
Cook potatoes for 5 to 8 minutes until spice is absorbed.
Serve the lamb madras with potatoes decorated with fresh coriander.