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1
Lay the lean mutton pieces and the dumba fat together on a heavy stone or wooden surface and begin pounding with a mallet, working steadily for 20–30 minutes minimum, turning and folding the mass back on itself between strokes, so the fat is driven into every part of the lean and the two become a single, uniform body rather than a crumbled mixture. Pound until the sound shifts — an initial slapping gives way to a low, wet squishing — because that change in register tells you the fibres have broken down completely and the fat has bound with the flesh. The mass is ready when it holds its shape under pressure without crumbling and feels near-silken to the touch, closer to a smooth dough than to minced meat.
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2
Divide the pounded meat into four equal portions. Wet your hands well before shaping each portion into a large, smooth, quenelle-like ball, so the surface stays cool and does not begin to tack and tear as you work. The surface of each ball must be entirely without cracks or seams, since any gap will open further in the sauce and allow the ball to split during the long simmer.
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3
Fry the sliced onions in a wide, heavy pan over moderate heat until deep golden and crisp throughout — this is the birista — then drain and grind them to a smooth paste, so their sweetness is concentrated and their raw sharpness is driven off before they enter the sauce. Return the paste to the pan, add the whisked yoghurt and the saffron with its soaking water, stir in the ground fennel, dried ginger, green and black cardamom, and salt, then warm over low heat, stirring continuously, so the yoghurt tightens gradually into a stable, cohesive sauce rather than breaking into curds and whey. The sauce is ready when it coats the back of a spoon and carries a uniform pale gold from the saffron; it must not boil at this stage.
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4
Lower the shaped gushtaba gently into the hot — not cold, not boiling — yoghurt sauce, because a sharp difference in temperature between the meatballs and the sauce will either shock-cook the outer layer into a tough skin or cause the sauce to split the moment the cold mass hits it. Maintain a gentle, steady simmer for 45–60 minutes, turning the meatballs once at the halfway point so the sauce penetrates evenly on all sides. The gushtaba is done when it yields at the slightest pressure from a spoon and the sauce has visibly soaked into the outer layer of each ball, leaving no pale, raw-looking crust.
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5
Taste the sauce for salt. Serve each meatball in its sauce immediately alongside plain rice or maash khichuri; the dish should arrive at the table pale, saffron-scented, and without any scorching or browning on the meatballs.