My 2 usual goto ideas:
Chicken Quarters (or bone in skin on chicken thighs sometimes) - I get these usually around $1/lb at the local grocery store, sometimes less. I just cook a pack of them (4 usually) on Sunday night or whenever, with some garlic, salt and rosemary usually at 375 for about 50-55 mins and lunch for the week is good to go however you want to take it (gallon bag will usually hold all of it, including drippings, but can be taken however you want). I often take a bag of spinach to go with it and just microwave at work together so the spinach soaks up some of the chicken juice (more tasty). Don’t waste the juice and fat drippings.
Make enough for dinner to take in leftovers. That’s pretty much that, and works well enough if you can do it (I’m in an interesting situation where my family is large enough that we need frequently need a certain size buy of food, but there’s usually a serving or two leftover that won’t be enough to feed everyone agian).
I keep a can of lo salt at work to put on everything (for the potassium, i put plenty of regular salt on stuff too).
Secondary ideas I’ve done:
Can of alfredo sauce, Mozzarrella Cheese (bag, block, string cheese stick, whatever you want), pepperoni (unsliced is cheaper per pound): Put in a bowl in sauce, cheese, pepperoni layers, microwave for a bit until cheese is melted or to your liking, and you got yourself a crustless pizza.
Eggs. Beat them in a cup or bowl, microwave for 30s-1min (or whatever is necessary) and there you go. I usually put some string cheese in during the process as well. The only drawback here is it can stick to the sides of whatever pretty bad if it cooks too long, but there are ways of dealing with that.
If you can find some tallow (I can’t around where I am anymore, unfortunately), you could get some tallow and ground beef and make some pemmican, that’s pretty good for a variety of on the go situations situations (meal, snack, hiking, whatever).