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Line a 20cm square baking tin with baking parchment and rub a little oil over the surface and sides. Bring the stock to the boil in a saucepan over a medium heat, then stir in the polenta. Cook, stirring continuously, for 5 mins, until thickened – the texture should be like loose mashed potato.
Add the rosemary, if using, and season with salt. Stir for another minute, then pour the polenta into the prepared tin and level the surface with the back of the spoon. Leave to cool, then chill for at least 30 mins until set firm. Can be made up to a day before serving and kept chilled in the fridge.
Heat the oven to 200C/180C fan/gas 6. Turn out the set polenta onto a chopping board and cut into thick chips. Arrange the chips on the baking tray in a single layer and brush each one with a little more oil. Bake for 25 mins, turning after 15 mins, until crisp.