Onions, leeks, garlic and shallots
Alliums are stringy in exactly the way this diet worries about, and usable in exactly one form.
The savoury base of most European cooking starts with an onion, so an outright ban on alliums would take a great deal of the book with it. It is worth being precise about what the actual problem is.
The issue
Not fibre in the abstract. Structure. The fibre in an onion or a leek runs in long parallel bundles, which is why a sliced onion holds together as rings and why a leek shreds into threads rather than breaking into pieces. Those strands survive chewing and stay recognisable a long way down the gut, and strands are what a narrowed bowel cannot manage.
Cooked far enough, that structure collapses entirely and there is nothing left to catch.
Where hospitals differ
The leaflet this book follows lists sausages with onions among foods to avoid and does not list onion as an allowed food. Several other trusts, St George's among them, allow well-cooked onion without much fuss. Leek is not on the allowed list of the leaflet this book follows and is allowed well cooked by St George's. Garlic is allowed outright by the leaflet this book follows, which is a useful gift, as long as it is cooked soft and there are no crisp fried pieces.
Spring onion is out everywhere. It is raw, it is stringy, and it is the garnish that every photograph of a bowl of noodles wants to add.
What this book does
Onion and shallot are used only cooked to melting and then either puréed into the sauce or strained out. Leek is white part only, sliced as finely as you can, cooked down to nothing, and then blended and sieved so that no thread gets through. Garlic is cooked soft and left in. Every recipe using onion or leek carries a line saying so and an alternative for readers who have been told to leave them out.
What you should do
Ask your team about well-cooked onion specifically, since it is the difference between a gravy that tastes of something and one that does not. If the answer is no, extra potato and a good stock will carry a dish surprisingly well, and every recipe here that uses an allium says what to put in its place.