add the mustard oil (or 3 tablespoon vegetable oil + 1 teaspoon dry mustard such as Colman's) to a pan and add in the meat. Cook until the meat is browned and starting to cook through, around 5 minutes then remove the meat and place aside (leave the oil)
add the onions to the pan and gently cook until caramelized, stirring frequently so they do not stick and burn
blend the caramelised onions and cashews with 1/2 cup water to form a paste and put aside
back to the pan add in your butter and chillies and fry for 2-3 mins stirring frequently
add the garlic and ginger paste, stir through and fry for 30-60 seconds
in with the tomato paste, blended onion and cashew paste and all of the spices, stir through and cook until the oil separates
add 125ml of hot water, stir through and allow to reduce by half
remove from the heat and add the cream, stir through and place back on the heat and cook until the oil separates again
add the remaining water as well as the meat. Stir and coat well and reduce until the consistency required and the meat is cooked through