Preheat oven to 375.
Scrub, dry, and slice the potatoes. Line them on the bottom of a 9x13” baking dish sprayed with cooking spray.
In a medium size saucepan over low-medium heat, melt the butter. As the butter melts, whisk in the flour one tablespoon at a time.
Measure one cup of milk and add it to the saucepan. Continue whisking until there are no more clumps. Add the final cup of milk as well as salt and pepper to taste. Continue whisking over low heat until the sauce begins to thicken. It should be similar to gravy, not too watery but not too thick.
When the sauce has reached the desired consistency, pour over the potato layer.
Place the baking dish with the potato and white sauce mixture in the oven. Bake for 25-35 minutes, or until they are bubbly and starting to soften. The potatoes don’t need to be fully cooked as they will go back in the oven for a bit with the meat mixture later on.
In a large skillet, brown the ground beef and onion. Season with salt, pepper, and minced garlic.
When the meat is finished cooking, slowly stir in the Cream of Mushroom soup, one soup can of milk, and the corn. Let the mixture simmer over low heat for five minutes or so.
When the potatoes are done cooking, remove them from the oven. Top with the meat mixture and finally the cheese. Bring the oven temperature down to 350 and return the hotdish (casserole) to the oven.
Bake for 20-25 minutes or until bubbly and cheese is melted.
