Bring a pan of salted water to the boil. Plunge in the sprouts and cook for 5 minutes, then drain and transfer to a bowl of very cold water (don't skip this step - it stops them overcooking). Let them sit for 1 minute, then drain and set aside.
Heat a large frying pan or saucepan over a high heat and fry the pancetta until crispy.
Add the sprouts and chestnuts (if using) and fry for about 5 minutes until they are hot through and coated in the fat. Season with salt and pepper, and the Parmesan (if using).
To make ahead:
The day before, cook and cool the sprouts to the end of step 1. Fry the pancetta until crispy. Put the cold sprouts and the frying pan with the cooked pancetta, covered, in the fridge. When you're ready to cook, put the frying pan on the hob over a high heat, until the oil that the pancetta released is very hot. Add the cold sprouts and the chestnuts (if using) and pan-fry for about 5 minutes until they are hot through and coated in oil. Top with Parmesan cheese (if using).
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