Remove the outer leaves from the sprouts, trim the base of each one and cut in half from top to bottom, through the root. Cook the prepared sprouts in boiling salted water for 3-4 minutes, or until nearly tender. Drain and refresh in a bowl of cold water to retain their bright-green colour.
Heat half of the butter in a large frying pan over a medium heat. When the butter is foaming, add the pancetta and fry for 3-4 minutes, or until crisp. Remove from the pan using a slotted spoon and set aside to drain on kitchen paper.
Heat the remaining butter in the same pan. Add the onion and chestnuts and fry for 8-10 minutes, or until the onion has softened. Stir in the cooked pancetta.
Drain the sprouts well, then stir them into the pancetta, chestnut and onion mixture. Season, to taste, with salt and freshly ground black pepper. Serve piping hot alongside roast turkey with all the trimmings.