Tip the dhal into a sieve and rinse under cold water several times until the water runs clear. Put in a large bowl or pot and cover in double their volume of warm water. Stir in the bicarbonate of soda and leave to soak at room temperature for at least 12 hrs.
Drain the dhal (but don’t rinse), tip into a pan and cover with 1.5 litres water. Add the sliced ginger, whole chilli and turmeric, then bring to the boil, skim off any scum that rises to the top and reduce the heat. Simmer for 1 hr until very tender, or cook in a pressure cooker for 20 mins. Remove the ginger and chilli.
Heat the butter in a separate pan over a medium-high heat until it has foamed up and turned nut-brown. Pour half into a small container and set aside. Return the pan and remaining butter to the heat and add the cumin seeds, fenugreek leaves, ground coriander, garlic, grated ginger and tomato purée and cook for 2 mins more. Stir the spice mixture into the cooked dhal, then add the kidney beans along with the liquid from the can.
Roughly mash the dhal mixture with a potato masher until some of the pulses are crushed. Cook, stirring, over a very low heat for 15-20 mins until the dhal is thick and creamy. Stir through most of the reserved brown butter, the garam marsala and half the cream. Tip the dhal into a serving dish and drizzle over the remaining cream and brown butter. Scatter over the chopped chilli and ginger matchsticks and serve.