If you're trying to economise, , then using whatever you have is the way to go. It's the basis of the many thick, main-course soups I still make in winter.
My essentials are first, onion and celery, chopped small and sweated in a little oil.
Good stock cubes, or homemade chicken stock from a carcass, or ham stock.
Plus, handful of red lentils or pearl barley or some of each, added with stock and simmered for at least 30 mins.
And/or a handful of orzo (rice shaped) pasta, added and simmered for as long as necessary. Or any other small pasta.
Any root veg - potato, carrot, swede, turnip, celeriac, etc. chopped small and simmered for at least 15 mins.
If I have any, a little bacon chopped small and fried with the onion. A good way to use up one lonely rasher that's getting a wee bit dried up.
Whatever green veg I have - cabbage, leeks, broccoli, cauliflower, sprouts, etc. chopped or shredded small and added for the last few minutes only.
Any bits of leftover ham or scraps of cooked chicken, also added last few minutes, just enough to heat through properly.
Ditto any leftover veg, mash, etc. from the fridge.
Plenty of black pepper.
Usually served with bread and plenty of grated cheese.
I started making these during our very broke years - we virtually lived on them for a while - dh still loves them.