Melt the butter over a low heat in a large, non-stick frying pan. Add the onion and cook for 10 mins until soft. Increase the heat to medium and stir in the bacon, sage and sausagemeat. Cook for 6-8 mins, breaking up the sausage with a wooden spoon, until starting to brown.
Tip in the sprouts and chestnuts, cover with a lid and cook, stirring occasionally, for 10 mins until the sprouts are tender. Keep them warm over a low heat until you are ready to serve. Just before serving, take off the lid, season, stir in the breadcrumbs and fry on high for 2 mins to toast them and get some colour on the sprouts.