I have done this lentils recipe quite a lot now, so here is a more detailed and illustrated version.
Heat a frying pan and add some chopped pancetta. There is no need to add any extra oil as there is sufficient fat in the pancetta.
Finely chop an onion.
Add the onion to the frying pan with the pancetta.
I softened in a frying pan, the finely chopped onion with the pancetta. When the onion was nicely soft, I added some sliced mushrooms.
I added some red wine and beef stock before simmering it down.
I use beef stock over chicken stock just because of the colour rather than flavour. I then reduced and reduced this.
Then I added some freshly chopped parsley for flavour and colour.
When I was happy with the sauce I whisked in some butter (with a fork as it happens rather than a whisk) and then added a packet of lentils.
You could use a can of lentils or even freshly cooked lentils. I needed to add a bit more stock at this point.