Spinach
With care — Allowed by some hospitals and not others, or allowed only prepared a particular way.
In one line: Cook it soft and purée it.
Spinach is a green leaf, and green leaves are the sort of thing this diet usually keeps out. What makes it different from cabbage or kale is how completely it collapses. A large bag of spinach wilts down to a few spoonfuls in a hot pan, and the leaf structure that would be a problem raw disappears in the process. Blended and sieved after that, there is nothing left with any shape to it.
The preparation
Cooked soft and puréed, not stirred in at the last moment and not used raw. Wilt it fully, blend it into the soup, then push the whole thing through a fine sieve, because a blender leaves flecks of leaf and stalk that the sieve catches. Pick out any thick stalks before cooking. Baby leaf spinach has fewer of them.
Where hospitals differ
Spinach is not on the allowed list of the leaflet this book follows. St George's classes well-cooked spinach as medium, meaning try it cautiously and see how you get on. This book uses it in one soup, cooked and sieved, and that recipe says the leaflet does not name it.
If you have been told to leave it out
The soup works on potato, stock and cream alone, and will be pale rather than green. Peeled courgette, deseeded, cooked soft and sieved, gives some of the same colour with a milder claim attached.