When dh and I were exceedingly skint, for quite some time we virtually lived on thick soups made from local, seasonal veg, and the cheapest cheese available locally (this was not in the U.K.) which was some processed Australian stuff called for some strange reason ‘Penguin’. And bread, of course. Any sort of meat or ‘proper’ cheese was unaffordable.
I still make quite a lot of such soups especially in the winter - often now with lentils/pearl barley etc. added. We still enjoy them.
When particularly hard up my mother had frequently made such soups with whatever she happened to have - hence her name of ‘Dustbin soup’, which is what we still call them.
I wouldn’t like to have to do it, but I’m confident that if I had to, I could feed us both extremely cheaply and reasonably healthily - Smartprice porridge and pasta (I usually buy that anyway), the cheapest UHT milk, lots of relatively cheap seasonal veg, the cheapest bread and cheese, and obviously no more free range eggs, etc.
I’m very thankful that I’m not likely to need to do it, though.