Preheat the oven to 180/350/gas 4
Slice the bottom off each sprout and peel off any tatty leaves. Tip into a baking tray. Add a tablespoon of your favourite cooking fat at this point if your bacon is very lean.
Top the sprouts with the bacon pieces and pop into the oven.
Cook for 15 minutes. Remove from the oven and gently but firmly squish each sprout with a potato masher - you're looking to open up the leaves of the sprout, not entirely flatten it!
Sprinkle with a little salt and pepper, add in the chestnuts, and toss everything around in the oil the bacon has released.
Roast for another 10 - 15 minutes or until the sprouts are cooked through and delicious.