Put a medium sized pan of water on to boil, add half a table spoon of sea salt.
Once boiling add the Brussels sprouts for a minute then drained them into a bowl of ice cold water.
Heat the rapeseed oil in a large frying pan, cut the shallots into slices and and sauté in the pan until golden with the sage leaves.
Then add the Brussels sprouts, season the pan with salt and pepper, lowering the heat cook for 6-8 minutes to almost fully cook the sprouts, stirring to prevent catching (add a splash of water if you’re concerned about the shallots burning).
Then add the chestnuts and continue cooking until the chestnuts are piping hot and serve in a warmed dish.