Put half the mustard seeds and all the cumin seeds, coriander seeds and cloves in a pestle and mortar or spice grinder and grind as finely as possible.
In a food processor, combine the ground spices, onion, garlic, ginger, vinegar, chilli, paprika, turmeric and water. Blend until smooth.
Rub the salt, pepper and 2 tbsp of the spice paste onto the shanks. Cover and refrigerate the shanks overnight or for at least 1 hour.
Heat a large saucepan with the oil and add the remaining mustard seeds. Once they start popping, add the rest of the spice paste and sauté for 5 minutes.
Add the shanks and sauté, turning often, until they’re browned. Add the stock and potatoes, cover and cook for 1-2 hours or until the meat falls off the bone.
Reserve the liquid and place it in a medium pot over a high heat. Allow it to reduce to approx. one-third of the original volume. If the gravy doesn’t thicken properly, combine 2 tsp cornflour and 3 tsp water together and whisk into the reduction. Continue to whisk until the cornflour has cooked out and the sauce has thickened. Season to taste.
Serve the shanks with the raita, gravy and rice.