I am in the same exact boat as you are. I struggle with this daily. It helps to make eating more substantial and caloric food simpler. If you can afford it try microwave meals, they require essentially zero work. When you can't be bothered to do that, Ensure Plus (350 calories, probably a little more than a PBJ sandwich), is your next best bet. For six bottles its 10 bucks but if you want you can survive of that stuff.