The duck is best cooked on a barbecue but if you don't have one, score the skins and sear the fatty side of the breasts until golden brown in a dry frying pan. Turn the duck over and seal the other side.
Transfer to a baking dish and cook for 12 minutes at 200°C/ Fan 180°C/Gas 6.
To make the lentils melt the butter and very slowly, with the lid on the pan, cook the onions and the bacon. It will take about 30 minutes and by that time the onions will be very soft and translucent.
Tip in the lentils, add the tomato puree, stock cube, red wine and lots of seasoning.
Bring to the boil and cook briskly for about 5 minutes until the flavours all come together and the liquid has reduced a little. Season well. You are looking for a rich, earthy taste, so you might need to add a dash of balsamic vinegar or some liquid stock.
The lentils can be prepared a day ahead or even frozen.
Slice the duck thinly and arrange on each plate on top of a mound of lentils.