Very open-plan soup recipe:
Dice 1 onion/leek and saute in a tiny smidge of olive oil, on very low heat. Toast a spice (if you're into that kind of thing) such as cumin or coriander or even caraway or fennel seed in there too. Add any or all of the following: diced carrots, a few celery sticks, a turnip/swede, 1/4 cabbage, sweet potato, pumpkin etc. Continue to sweat over low heat, stirring occasionally. Add stock to cover vegetables. Add lentils (can use red, green, brown, split peas, anything that doesn't need soaking) and bring to the boil. Continue at medium heat until lentils and all vegies are soft. At this point you could add spinach, nut butter, tahini, etc if you fancy. Remove from heat. Blend, if you like a silky soup. Alternatively, leave it chunky. Season as required (do this last so it doesn't slow down your lentils cooking) and eat. Yum yum yum yum yum.