Dry fry coriander and cumin seeds over high heat until fragrant, then grind with other spices into a powder.
Preheat the oven to 150°C and season the lamb shoulder with salt and pepper.
Sear the lamb in oil in a large flameproof casserole for 2 minutes per side until brown.
Fry onions and chillies in the pan for 5 minutes, then add the ground spices, garlic, and ginger.
Stir in cardamom, curry leaves, tamarind, tomatoes, and beef stock, then bring to a simmer.
Cover the casserole and roast in the oven for 3 to 4 hours until the lamb is tender.
Boil potato cubes in salted water for 12 to 15 minutes until tender, then drain.
Sauté onion, garlic, and Bombay spices in vegetable oil in a large frying pan.
Add chopped tomatoes to the potatoes spice base and bring to a gentle simmer.
Toss the cooked potatoes in the sauce for 5 minutes and stir in fresh herbs and seasoning.
Uncover the lamb, baste with sauce, cook for 5 minutes, then stir in fresh herbs.
Serve the spiced lamb alongside a portion of Bombay potatoes.