Why this recipe works
Kosha means slow-cooked and reduced, and that long, patient bhuna is the whole dish. Tough mutton is browned and then cooked low for a long time with onion and yogurt, so the collagen melts to gelatin and the meat turns tender while the gravy reduces to a dark, glossy coat. There is little water, just the slow release of flavour and the mustard-oil richness.
Ingredients
Method
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1
Marinate the mutton in yogurt, ginger-garlic and Kashmiri chilli for a few hours.
Why yogurt: the acid and enzymes begin tenderising the tough meat and season it deep before the long cook.
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2
Brown the onions and mutton well in mustard oil with the whole spices.
Why brown it: searing builds a deep, savoury crust (Maillard) that the long bhuna then concentrates into the dark, intense gravy.
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3
Cook low and slow, stirring often, for well over an hour, adding only a splash of water when needed.
The soul of it: this long, patient bhuna is where kosha mangsho is made, the collagen slowly melts to gelatin, the meat turns tender, and the gravy reduces to a glossy, clinging coat.
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4
Finish with a pinch of sugar and garam masala when the oil separates and the gravy is dark and thick.
An honest word
Kosha mangsho is a rich, festive dish: mutton is a fattier meat and the slow cook uses a fair amount of mustard oil. It is worth every bit as an occasional Sunday feast. Lighter notes: use leaner cuts, trim visible fat and skim the surface oil after cooking, and keep the mustard oil modest. The tenderness comes from the patient bhuna, not the fat.