As a few people have suggested, cooking up a big batch of something to give you a few portions is something I do a lot, both for lunch & evening meals. One favourite of mine is to have a chilli, and then eat it using tortilla wraps rather than cook rice, which is more effort.
A very easy thing I do a lot is to get some frozen spicy Mexican bean burgers from Tesco (the other supermarkets will probably do it too), then pop one in the oven with some sweet potato wedges drizzled with olive oil (sweet potato is much healthier than normal potatoes). Have some soured cream, guacamole or mayo with it for a for extra calories.
Something I’ve done for my lunches this week is chopped some pepper & cucumber, and mixed it with some tinned mackerel (twice the calories of tuna, and healthier too), mayonnaise & pesto. You can easily prepare all that while sitting down at the table. I’ll then stuff pitta bread or tortilla wraps with it.
And one final one - roughly chop a load of Mediterranean vegetables (aubergine, peppers, red onion, courgette etc), drizzle with oil & chuck it in the oven for half an hour. Then mix it with some pasta, smoked salmon (super healthy, & very high in calories. I buy smoked salmon trimmings, which are loads cheaper), and pesto. For an even easier option, you can just buy jars of things like roasted peppers, tomatoes, artichokes from the supermarket, and use them instead. The leftovers can be easily reheated.
By the way, did you know you can get perch stools to help in the kitchen? These are raised seats you can sit on while you prepare things. If you ask to be referred to an occupational therapist, they’ll be able to provide one for you. Or you could buy one yourself from any mobility shop.