Method

Roast the cumin and coriander seeds in a dry frying pan until fragrant and then grind in a pestle and mortar or a spice grinder. This is known as 'dhana-jeeru' and features frequently in Indian cooking.
Melt the butter with the oil in a large pan and add the onion, cloves, cassia (or cinnamon), and peppercorns. Cook, stirring occasionally, over a medium high heat until the onion is well browned.
Discard the cinnamon. Add 1 tablespoon of the dhana-jeeru mix, the garam masala, and the turmeric and fry for about 30 seconds stirring.
Add the tin of tomatoes and break them up with a spatula. Peel and mince the garlic and ginger and add to the pan with the chillis.
Cook the mixture over a medium heat for about 10 minutes, until the oil separates from the mixture at the edges.
Add the minced lamb and break up and mix into the mixture. Cover and cook over a low simmer for 35-45 minutes, adding the peas 10 minutes before the end. Check the seasoning and add more salt and garam masala if reqired.
Like most curries, this dish is best when cooled then reheated to allow the flavours to infuse, but this is optional. Reheat in the pan and serve with rice and / or chapattis.