When my mother was in hospital before Christmas, she responded very well to their 'soft platter' menu which is basically a plateful of finger food.... chips, sliced pepper (sometimes works, especially if it's red), bits of quiche, piece of cheese in finger form (easily picked up, not crumbly) and so on. Basically, anything that can be presented simply and is easy to eat and tasty. Mum even ate a whole hard boiled egg, biting off it like fruit. And if a clementine is sweet enough (always a lottery) she'll eat that if I separate the segment, strip off any obvious pith, and leave them withing reach. Of course she may also turn them into sticky mush, but such is dementia life!
My go-to backup 'meal' is mashed banana with pieces of cake chopped up in it, covered in instant custard with a diet shake added for extra nutrition. If I can get her to eat the first mouthful (often the biggest challenge!) the rest usually goes down. If not, it gets served up stirred into porridge the following day.
Our porridge is dementia-friendly... make it with milk, stir in some Coffee Mate to make it richer, add plenty of drinking chocolate until it's more pudding than porridge, and serve. Not ideal, but those porridge oats keep things moving downstairs and I know myself that if I eat them regularly (even if in unhealthy form!) my digestive system is healthier.
Angel delight is another good way to disguise something a bit healthier... like more mashed banana (apparently one of the few foodstuffs you can eat on its own for a long period without suffering too much too quickly) or tinned fruit And I put a diet shake or Complan in that too, to boost it.
I always feel really sorry for those who have diabetes to contend with as well. Must be an absolute nightmare.