Lentil soup is easier to make than most bean soups because you don’t have to soak the lentils beforehand and lentils cook very quickly—in 45 minutes to an hour or even less.
Even though you can make a lentil soup with just a little onion and some broth, I always like to include something smoky such as bacon or ham. If I have it, I also like to add confit of duck. Although I prefer pureed lentil soup, it’s also fine with the lentils left whole.
The most commonly available lentils are the regular brown lentils sold in plastic bags in supermarkets. These are perfectly good, but the next time you get to a fancy food store, buy a bag of tiny de puy lentils from France or the bright orange Indian or Egyptian lentils. Different lentils take different cooking times, so start checking them after 20 minutes—take a nibble; they should be completely soft.
Although most packaged lentils are free of debris, it’s a good idea to pour them out on a baking sheet and check them for pebbles. Remember when cooking lentil soup to stir it every once in a while to prevent sticking and scalding.