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1
Grease two or three flat surfaces — large steel plates, a marble slab, or thalis — thoroughly with oil before any heat is applied to the batter, so that when the batter is ready you lose no time hunting for equipment and the oil has a moment to settle into an even film. Set them on a cool, level surface within arm's reach of the stove. The spreading window once the batter is ready is under two minutes, so this preparation must be complete before cooking begins.
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2
Whisk the besan, yoghurt, water, turmeric, and salt together until completely lump-free, because any lump that enters the pan will stay a lump and show in the finished sheet. The batter should be fluid and coat a spoon without lumps. Pour into a heavy-based pan.
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3
Cook the batter over medium heat, stirring constantly with a flat spatula — scraping the base and sides without pause — so the batter thickens uniformly rather than scorching in patches before the centre is ready. Test readiness by dropping a small spoonful onto one of the greased plates; allow it to cool for 30 seconds, then attempt to roll it. This is where the dish lives or dies: if the cooled film rolls cleanly without cracking and without sticking, the batter has reached the correct end-point; if it tears, cook further; if it cracks, it is marginally over-cooked but may still be usable if spread immediately, and the batter in the pan should be thick enough that a line drawn through it with the spatula holds its shape and no liquid seeps back in.
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4
Remove the pan from the heat immediately and, working quickly, ladle the hot batter onto the greased surfaces and spread it to a layer of 2–3 mm using the back of a large spoon or offset spatula, using long, decisive strokes, so the sheet is as thin and even as possible — an uneven or thick layer will crack on rolling. The batter will become unspreadable as it cools, so do not pause between ladling and spreading each surface.
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5
Allow the spread sheets to cool completely at room temperature so the structure firms all the way through rather than remaining soft at the centre where it would tear when rolled. The sheet is set when it lifts cleanly from the surface with a palette knife at the edge and no longer feels tacky to the lightest touch of a fingertip.
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6
Using a knife or pastry scraper, cut the sheet into strips approximately 5–6 cm wide, keeping the cuts clean and straight so the rolls close evenly along their length. Starting from one short edge, roll each strip into a tight cylinder — firm, compact, and holding its shape without springing open. Arrange the rolls on a serving plate.
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7
Heat the oil in a small pan over medium-high heat, then add the mustard seeds and cover loosely so the seeds pop without scattering across the stove. When the mustard seeds pop audibly and the sesame seeds begin to colour to pale gold, pour the tadka immediately over the arranged rolls so the sizzling fat carries the fragrance of the spices into every crevice of the rolls.