My usual veg soup is one of dh’s favourites.
Fry some chopped onion gently for a few minutes until soft, not brown, add stock (homemade chicken is nice, otherwise a chicken or veg stock cube) add any or all of chopped celery (essential IMO for flavour) carrot, swede, celeriac, potato if you want it more filling. Simmer for a good half hour.
Then add any green veg, chopped - cabbage, leeks, quartered sprouts, broccoli, cauliflower, bring back to boil and simmer for just a few minutes. Some chopped parsley is a plus.,
For a more substantial, main meal soup, chuck in a handful of red lentils or pearl barley or both, with the root veg, and simmer for 10-15 mins longer until cooked, before adding green veg.
Taste and add seasoning or another stock cube if needed, and plenty of black pepper.
We usually have it with grated cheese on top. It usually contains around 10 different plant items.