In a large pan melt the butter and then add chopped onions, carrot and celery. Tear in the rosemary and a couple of bay leaves and after a few minutes add the chopped garlic. When all have nicely softened, tip in the lentils and pour over 1litre/1¾ pints of vegetable stock. Bring to the boil and then leave to simmer whilst you tend to the pumpkin.
Using a variety of pumpkins and squashes gives the stew a more complex flavour. Here I use acorn squash and Cinderella, but you want to end up with about 600g/1lb 5oz of peeled pumpkin in large chunks. Add them to the stew, season and then pour over enough water to just cover all the ingredients. Put a lid on the pan and leave to simmer for 30-40 minutes.
When the stew is almost ready, add the chopped parsley.
To make the stew creamier, remove a small bowlful to a food processor and blitz it with 200ml/2fl oz of stock. Pour it back in and the stew becomes instantly more velvety.
Serve the stew in bowls, finishing each helping off with a cooling spoonful of sour cream.