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1
Place the white poppy seeds in a bowl, cover generously with cold water, and leave to soak for a minimum of 30 minutes, so the seeds soften from the inside out and will grind to a smooth, dense paste rather than a gritty one. Drain, then grind with a small splash of water until no grit can be felt when the paste is pressed between the fingers. For the quick weekday method, skip grinding: the soaked, drained seeds may go directly into the pan at Step 3, where the steam from a covered pan will work them into a paste against the potato.
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2
Heat the mustard oil in a heavy pan over medium heat until it loses its raw sharpness and shimmers, because raw mustard oil carries a harsh, acrid bite that would dominate the delicate posto. Add the potato pieces and salt, stir to coat in the oil, and cook uncovered over medium heat, stirring occasionally, until the surfaces are just beginning to colour and a skewer meets resistance at the centre — the potato must be approximately half-cooked before the paste goes in.
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3
Reduce the heat to low, add the posto paste — or the soaked, drained whole seeds for the weekday method — and the green chillies, then stir once to distribute the paste among the potato pieces. Cover the pan immediately, so the paste steams gently onto the potato rather than seizing and scorching on the hot pan surface. This is where the dish lives or dies: high heat or an uncovered pan at this stage will burn the posto before the potato is cooked through, leaving a bitter, darkened crust instead of the clean, milky coating that defines the dish.
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4
Cook covered on the lowest heat for 10–12 minutes, then lift the lid and press a piece of potato — it should yield completely, and each piece should wear a thick, matte white-cream crust with no liquid remaining in the pan. If any moisture persists, replace the lid and continue for a further 2–3 minutes, giving the steam time to be absorbed back into the paste rather than leaving the posto wet and loose.