When helping to care for a very elderly relative, I made and froze tiny portions of 'traditional' dishes such as beef or lamb stew, but with the pieces cut very small, and a little bit of leek rather than onion (it's more digestible) . Also mince - simply cooked - and, as you suggest, fish pie. I also made little pots of bland things such as parsley sauce or little pats of steamed spinach to go with a tiny piece of microwaved fish, but your relative might not be able to assemble those. Nutrition also came from petits filous, small helpings of yoghurt, rice pudding, baked custard, bread and butter pudding, pureed apples or apricots. Mashed swede and carrot with butter also provided calories. One nurse joked - and she was correct, in a way - 'add grated cheese to everything!' In other words, smuggle extra food value in where you can.
My aged relative was fond of soup and found it easy to handle, and would sometimes eat it with a quarter of a cheese or very mild ham sandwich (soft white bread, no crusts) or a little triangle of bread and butter. This sounds dreadful - and she would never have countenanced this when younger and well - but she sometimes would enjoy a little Heinz tinned pasta - spaghetti, ravioli, macaroni cheese etc. So perhaps the texture of frozen pasta would not be too much of a problem?
We found that salads or almost anything raw - except sliced banana and occasionally a few well-sugared berries - were not a good idea. My relative found the crisp texture hard to cope with, and also fiddly and difficult to eat. Peas were a bit of a struggle, too. Veg were mostly (as above) carrots, swede, parsnip, well-cooked broccoli mashed, spinach, a few green beans. I put celery as well as leeks in stews. In extremis, mashed potato, a few baked beans and grated cheese made a meal.
The PP who suggested children's meals had a good idea. My relative felt overwhelmed by a normal portion. They also really disliked Wiltshire Farm Foods, but they had been a good plain cook who used fresh ingredients and never really been fond of ready meals.
I also agree with PP who suggested fruit cake, gingerbread etc - anything soft textured that will store and can be cut into small slices. My relative liked madeira cake and lemon drizzle, too.
There are 'meals in a drink' - the brand I remember was called Fortisip but I expect there are others. They were prescribed by relative's GP. My relative did not like them but nutritionally, they're v. good.