If you've got potatoes you just need a masher, some salt, some oil/butter, some water/milk, and a bowl. Gotta eyeball the consistency to figure out how much you need of the liquid, the rest is for taste. Cheap as hell but it'll get you fat. Peeling is optional.

When we were broke, eggs were always a good standby. We didn't use a microwave, but I have had microwaved eggs and they're decent enough. https://www.foodnetwork.com/how-to/packages/food-network-essentials/how-to-cook-an-egg-in-the-microwave

You can have mug cake for dessert: https://www.allrecipes.com/recipe/241038/microwave-chocolate-mug-cake/

Microwave potatoes, veggies and ham with a little butter. If you add warm milk afterwards, it's a soup.

Tortillas with cheese and whatever other topping. They get kind of weird crispy, but it's better than a lunchable.