This is a recipe from Madhur Jaffrey's Curry Easy. I've given her version, with my comments in italics; my version included three pieces of neck of lamb rather than lamb shanks, with the other quantities scaled down accordingly.
In spite of my reservations about the recipe as it stands, I'd like to give it another go. The sauce has an appealing zing and fragrance, thanks to the large quantities of ginger, cardamom, and coriander.
4 medium lamb shanks
Salt and freshly ground black pepper
350ml Greek or other thick yoghurt
6 cloves garlic, peeled and chopped
7.5cm piece fresh ginger, peeled and chopped
3tbsp ground coriander
2tsp ground cumin
1/2tsp cayenne pepper
4tbsp olive or rapeseed oil
10 cardomom pods
1/2tsp black peppercorns
2 x 7.5cm cinnamon sticks
1/2tsp cloves
1 medium onion, peeled and sliced into fine half-rings
Pat the lamb shanks dry with kitchen paper. Sprinkle all over with 1/2tsp salt and lots of freshly ground black pepper. I seasoned my lamb with a little salt only; ground pepper, stewed, can become bitter.
Preheat the oven to 160C/gas mark 3. I baked my curry at this setting for 30 minutes, by which time it was simmering. At that point, I turned down the oven to gas mark S/130C.
Put 120ml of the yoghurt, the garlic and ginger into a blender, and blend until smooth. Add the coriander, cumin, cayenne pepper, one and a half tsp of salt, and the rest of the yoghurt. Blend to mix. I used whole coriander and cumin, lightly toasted in a dry saucepan and ground with a pestle and mortar.
Put the oil [this is a lot, given the fattiness of the meat; I used only a tbsp, of sunflower oil] in a casserole large enough to hold the meat, and place over a medium heat. When hot, put in the cardamom, peppercorns, cinnamon, cloves, and the shanks, and brown them on one side. Turn them over, dropping the onion slices in the spaces between them. Brown both meat and onions, moving them around the pan as you need to.
This is a tricky way to brown the meat and onion. You are likely to find, no matter how carefully you stir, that bits of onion and spice catch on the casserole and burn. I'd be inclined to coat the meat with a little oil and brown it on a grill pan, quickly, while frying the onions and spices separately in the casserole. Or, if the liquid is not going to cover the meat, to allow the browning process to take place inside the covered casserole.
Add the paste from the blender and 250ml water. I'd prefer to use stock, or even, dare I say it, a diluted, halved stock cube. Bring to a simmer. Cover the casserole, and place it in the oven for three hours, turning the shanks every 30 minutes.
The yoghurt split, of course. One way of trying to prevent the separation - but not one I can guarantee, having not tried it for a while - is to add a dstp of flour (gram or cornflour, preferably) to the yoghurt mix. Another is to add the garlic, ginger and spices to the casserole with the water or stock, while holding back the yoghurt to the end. Take the cooked dish out of the oven, let the simmering subside, add a little sauce to the yoghurt, then add some more, and then pour this yoghurt and sauce mixture into the casserole. If you want to warm the dish again, make sure you do not allow the sauce to bubble.