Onions, yellow lentils (mung dal, available in the world foods aisle at Asda (huge bag for £3) or any Asian shop.
Gently fry finely chopped onions, add water, a good handful of lentils and stock cubes - chicken or veg, I often use both.
Add any root veg chopped small - carrots, potatoes, celeriac, swede, turnip.
Simmer all for at least half an hour, until lentils and veg are soft.
Add any green veg - broccoli, cauliflower, any cabbage - all chopped or shredded small, halved or quartered sprouts peas - just for the last few minutes.
Will keep fine in the fridge.
You can add a snipped rasher of bacon at the onion-frying stage.
An added filler at the pre simmer stage is pearl barley. Orzo pasta also good, but for the last 10 or so minutes only.
Served with grated cheese, this is a ‘main meal’ soup.
I also use red lentils, but lately do prefer the yellow ones for soup.