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1
In a sieve, rinse the urad dal in a couple of changes of cold water, until the water runs clear, then drain and put into a deep pan-they will double in volume while soaking. Cover with a generous amount of just-boiled water and leave them to soak for at least 6 and up to 24 hours.
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2
Once they've finished soaking, rinse, drain, and put back into the pan. Cover them with cold water and bring to the boil, then continue boiling for 45 minutes. Scrape off any scum that forms on the top and discard.
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3
Meanwhile, put 4 tablespoons of butter into a frying pan on a medium heat. When it starts to foam, add the onions and cook for 15 minutes, then add the ginger and garlic. Fry for another 5 minutes. It's not worth skimping on the time here: The longer you cook these (without burning), the more flavorful your dal will be. Add the tomato paste, salt, and chili powder, stir well, then take off the heat and set aside.
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4
Once boiled, the dal should be soft enough to crush against the side of the pan. When it is done, keep enough water in the pan to just cover them and drain the rest off. Add the onion and tomato mixture and the milk, bring to the boil, then turn the heat down to a simmer.
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5
Stir every now and then for around 1 1/2 hours. If the sauce starts to run low over time, top it up with an equal mixture of whole milk and water. The sauce will start to turn darker, richer and creamier. If the dal is not rich and dark after 1 1/2 hours, give it some more time: You can't do this dish any harm by cooking it for a little longer.
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6
Taste, adjust the salt and chili if necessary, and add the remaining butter just before serving and stir. Serve alongside rice or hot fluffy naan.
