I just use whatever I have, but always onion and celery, chopped small and fried gently for a few mins, then add water and any root veg - potatoes, carrots, swede, turnip, etc., again chopped small, plus a handful of red lentils/pearl barley/both.
If there’s one lonely, dried up rasher of bacon in the fridge, I’ll snip it up and fry with the onion and celery.
I simmer that lot for 30-45 mins, with a couple of chicken or veg stock cubes - or proper chicken stock if I have any, though that will need salt and pepper added.
Sometimes I add a handful of small pasta for the last 10 mins or so. Finally for the last few mins, any green veg - cabbage, sprouts, cauliflower, broccoli, etc., all chopped small.
If I have any final scraps of roast chicken, they’ll go in for the last few minutes, too.
This is a main meal in our house - dh loves it! - usually served with plenty of good strong grated Cheddar to sprinkle on top.