Step 1

Put the onion, lemon juice and a quarter-teaspoon of salt in a small bowl and leave to pickle and soften.

Step 2

Line a 28cm x 18cm baking dish with a sheet of baking paper large enough to cover the base and sides.

Step 3

Put two teaspoons of the chaat masala, all the turmeric, olive oil, 700ml water, a teaspoon of salt and a good grind of pepper in a medium saucepan on a medium-high heat. Bring to a simmer, then turn the heat to low. Stir the gram flour and polenta in a bowl to combine, then slowly pour into the water, whisking continuously to ensure there are no big lumps (there will still be a few small ones). Cook for four minutes, whisking vigorously, until the mixture thickens and starts to pull away from the sides of the pan, then quickly transfer to the prepared tin and smooth out the top with a spatula. Leave to cool for about 10 minutes, then cover and refrigerate for about 30 minutes, until set completely.

Step 4

Meanwhile, make the mayo. Set a sieve over a bowl and drain the tin of chickpeas. Separately measure out 50g chickpeas (save the rest for another use) and 40g of their water (discard the rest), then put in a food processor with the garlic, mustard, lemon juice and an eighth of a teaspoon of salt, and blitz smooth, scraping the sides every now and then as you go. With the motor running, very slowly drizzle in the sunflower oil, until the mixture comes together into a loose, mayonnaise-like consistency, then transfer to a small bowl (and refrigerate if you’re making this ahead of time).

Step 5

Use the paper lining to lift the chickpea mixture out of its tin and on to a board. Trim the edges slightly, to give you a neat rectangle, then cut lengthways into 12 1.5cm-thick slices. Now cut again widthways in half, to leave you you 24 pieces in total.

Step 6

Heat the sunflower oil in a medium saucepan on a medium-high heat. Once very hot, fry the chickpea pieces in about three batches for about five minutes a batch, until golden and crisp on the outside, using a slotted spoon to move them around a little as they cook. Transfer to a tray lined with absorbent paper while you cook the rest. Once all the pieces are cooked, sprinkle lightly with salt.

Step 7

Stir the coriander and chillies into the pickled onion bowl. To serve, pile the chickpea chips on to a plate and sprinkle all over with the remaining quarter-teaspoon of chaat masala. Spoon the onion mixture to one side, put the mayo bowl on the plate and serve warm.