I’ve got a bunch of frozen mashed potatoes pre-divided into meal-sized tupperware. Microwave one of those and it’s quite hearty.

I’ve also got a rice cooker and it’s super easy to make something both substantial and tasty with one of those, dump in the rice and water and then add a tin of condensed soup as well. Push the button and come back later to dump it onto a plate. I’ve found most kinds of condensed soup work well, though avoid anything with “cream of” in the title as those can end up unpleasantly goopy.