Put the oil in a large saucepan on a medium heat, then fry the bacon, if using, until it browns lightly and the fat begins to render.
Add the onions, carrots and swede, and fry, stirring regularly, until the onion is golden and the other vegetables are starting to soften.
Stir in the lentils until they’re well coated in fat, then add the mace, a pinch of salt, a good grind of pepper and the bay leaf. Add the chicken stock.
Bring to a simmer, then turn down the heat, partially cover the pan and leave to cook for about 45 minutes, until the vegetables are soft and the lentils have mostly broken down.
Remove and discard the bay leaf. Scoop out about a third of the soup, whizz it to a puree, then stir back into the pot. Adjust the seasoning to taste, and enjoy.