Puy lentils take longer to cook than is stated on the packets and in most recipes, in my experience. Soaking speeds the process of tenderising them. It also ensures that they absorb less liquid while cooking: you can barely cover them, and be reasonably confident that they will soften. If your water is hard, you may get better results if you use filtered water.
1 clove garlic, chopped
1 medium onion, chopped
150g Puy lentils, soaked for two hours or longer
Handful of flat-leaf parsley, chopped
Warm about 2tbsps of olive oil in a heavy pan, throw in the garlic, let it sizzle gently for a minute, and throw in the onion. Cook over a low heat until the onion starts to soften - five to 10 minutes. If the onion threatens to catch, add more oil.
Drain and rinse the lentils. Tip them into the pan, and pour in just enough stock to cover. Bring to a simmer, put a lid on the pan, and simmer gently until the lentils soften. It may take 20 to 30 minutes. Add more stock if the top layer of lentils becomes exposed.
Once the lentils are soft or nearly there, you may want to uncover the pan and turn up the heat slightly, to get a less soupy consistency. You'll need to stir the lentils regularly, because they'll catch as the liquid evaporates and the stew thickens.
Add salt to taste. (But I must admit that I've never properly tested the theory that salt added at an earlier stage compromises the texture of lentils, as it does dried beans.)
Cut the morcilla into fork-sized pieces, submerge them in the lentils, and give them five minutes to warm through. Stir in the chopped parsley. The stew would benefit from plenty of pepper; I stirred harissa into mine.