When trying to gain weight (or simply not go minus as in your case) the method will be the opposite of what is needed to lose weight. I have helped someone with this in the past and what I saw as his greatest trouble was that he would get too full to eat more very quickly. I asked about his diet and it was just full of foods which are very filling without actually containing many calories. Lots of fruits and vegetables with almost no carb and no fat.
So really what you need are easily digestible and not too filling calorie rich ingredients. Think lots of grains and fat. Buttered potatoes instead of air fried potatoes. Carrots instead of lettuce. White pasta over whole grain pasta. Cream or mayo based sauce instead of a stock/water based sauce etc. However still try to eat healthy. If going for bread take the white bread without added sugar for example. And still include vegetables but don’t make them over ⅓ of your plate. I have read many success stories with adding heavy amounts of dairy to the diet which makes sense since milk is there to grow a calf as fast as possible. Drinking a package of milk a day is almost a miracle cure to being underweight if you can stomach it. In fact the medical food packs they give to malnourished children are dairy based. Consider it if your diet allows it.
However what specific meals which are convenient to bring I don’t have many ideas. But I hope this mode of thinking will help at least a bit. It has to be a big portion that you can actually stomach. Think about which foods you seem to be able to eat a huge amount of and then narrow those down to the most calorie rich. They also have to not clog your stomach for the whole rest of the day so being easy to digest is also key.
Well what is a mental illness? Historically you had an illness if something caused you to not function properly at the things society expected you to function in. You can’t remember things clearly anymore and it affects your career or social life? You now are considered to have the mental illness of dementia. Are you just slightly forgetful but can still function fine? You are not ill, instead that forgetfulness is just a character trait of yours. And it goes on like this. Back in the day the expectations to marry the opposite sex and have children was huge so being gay was considered a mental illness. Today it’s more accepted and being gay is just part of who you are (in some places at least). And it goes on like this.
I think not much has changed in this regard. If a certain mental affliction doesn’t affect your social or work life in a meaningful way it’s not really considered an illness. So if you take this as a definition of being mentally healthy there are certainly lots of people out there who have no affliction which seriously hurt their current social or work life. However this of course doesn’t mean that these people are completely free from any mental affliction, it just means these mental afflictions are not considered serious in today’s society.
And these things change all the time as society’s expectations change. Back in the day when only a small minority could read and write there was not a diagnosis for dyslexia. Now that reading ability is expected of everyone in today’s society we have defined dyslexia. As new societal expectations come and go the list of diagnosable illnesses will certainly have things getting removed and added. Let’s say theoretically some people can’t handle zero gravity, today this is not relevant for the vast majority of people so there is not any diagnosis for such an illness. But if more and more people are expected to live in zero G then such a diagnosis would be sure to be defined if such an affliction exists.
Is this definition reasonable? Well that’s a completely different question. But as I see it this is how it is generally defined right now.