Lentil Stew is cheap, tasty, and freezable.
I don’t know what it is called, but lentils & rice with onions and sausage is fantastic, extremely quick. Add a dash of apple cider vinegar at the very end and spice the lentils with cumin while they cook.
Use an onion, garlic, and ground beef to spice up bottled pasta sauce, then freeze what you don’t use immediately.
Don’t make instant oatmeal—instead get steel cut oats from the bulk section of your grocery. Make it once a week (the oatmeal will stay good in the fridge) and add a bit of sugar and handful of whatever nuts and fruit or dried fruits you have on hand or can get for cheap. My favorite is to add brown sugar, cinnamon, apple, and walnuts. It is so much tastier.
Stir fry is another low-cost item. Get whatever meat is on sale and whatever fresh or frozen veggies are also on sale. You can get a generic stir fry sauce or look up recipes online. Last night, for example, we had skirt steak, snow peas, and onion stir fry. It’s quick, easy to make for only two, and super tasty.
To address the health issue—mashed potatoes are usually heavy on cream, butter, and salt. Creamed corn also usually has a high level of fat—not only from the cream but also from butter. You don’t want to forget your green veggies, either. To get more green veg in, get some frozen spinach and mix it in with your pasta sauce.
A lot of cooking on the cheap is being flexible enough to know what generally goes well together and taking advantage of what’s on sale. For example, if you plan on stir fry, you can pick up the sale meat and sale veg to go in it. If you plan on tacos or fajitas, again, you can get the best meat at the store while sticking to the general meal plan.