Remove the first couple of layers of the calçots and clean them thoroughly, the way you would leeks, to get rid of any dirt.
Bring a large pan of slightly salted water to the boil and blanch the calçots until soft, then remove them to a large bowl of iced water.
Make the tempura batter by whisking all the ingredients together, then cover and chill in the fridge until cold (the colder the tempura batter, the more crunchy cracks it will make when it comes into contact with the hot oil).
Cut each calçot into three, so they fit in your pan. Pour a couple of centimetres of rapeseed oil into a high-sided pan or sauté pan on a medium heat and leave to get hot.
Dip the calçots into the batter, then slowly lower them into the pan, stroking the bottom forward into the oil a few times before laying the calçot down, away from you. Doing this means the bottom of the calçot builds up a crust so it won’t stick to the pan and also ensures that the oil doesn’t splash towards you. Repeat, ensuring that the calçots have enough space to become crispy and golden brown (you will probably need to do three or so batches, depending on the size of your pan). Cook for around a minute and a half, then remove with a slotted spoon and drain on kitchen paper.
Season with salt and pepper and serve with the smoked chilli alioli.