In a large, deep frying pan or casserole over a medium heat, fry the onion, celery, garlic, bay, chilli and a pinch of salt in the olive oil until the vegetables are softish and translucent.
Add the lentils, stir for a minute, add 1.2 litres water, bring to a boil, then reduce to a simmer.
After 15 minutes, add the potato, and simmer until the lentils are tender but still holding their shape; the idea is that much of the water has evaporated, but not all, so the lentils are still slightly soupy. Add more water if the pan looks dry.
In the last few minutes of cooking, add the spinach or escarole and wilt down. Serve with some olive oil poured over the top.
If you are serving with sausages, you have two options. The first is to cook them in a separate pan and then serve on top. The second is to start them off in another pan and then add them along with the potato so they finish cooking with the other ingredients.