Poaching and steaming
Gentle wet heat gives tender protein without a browned crust, which is what a sore mouth and a cautious gut both want.
Meat, fish and eggs contain no fibre at all, so nothing about them is restricted here. What can still cause trouble is texture. A dry, over-roasted chicken breast is tough and stringy, and a crisp browned edge is a hard fragment in a mouth that is sore from treatment. Wet heat solves both. It also happens to be the most forgiving way to cook fish.
How to do it
Poaching means cooking submerged in liquid held below a simmer, at the point where the surface shivers and does not bubble. Steaming means cooking in the vapour above it. Both are gentle, and both stop the protein tightening and squeezing out its moisture.
Keep the liquid tasting of something. Poached chicken in salted water is dull, and poached chicken in clear stock with a bay leaf lifted out afterwards is not.
Salmon in a paper parcel with butter, lemon and a splash of wine is really steaming in disguise, and the parcel keeps every bit of the juice.
Take fish off the heat a shade earlier than you think. It carries on cooking in its own warmth.
The cue
White fish is done when the flakes have just parted and the flesh has turned from glassy to opaque all the way to the middle. Salmon is done a little before that, when the centre is still slightly darker. Poached chicken is done when the juices run clear and a slice pulls apart with a fork rather than needing a knife. If you have to saw it, it went too far.
Common mistakes
Letting the poaching liquid boil, which toughens and shreds. Cooking straight from the fridge, so the outside overcooks while the middle catches up. Throwing away the poaching liquid, which is stock now and belongs in a soup. And leaving the skin on, which stays on the plate rather than on the fish.