In a large mixing bowl, thoroughly combine the ground beef, milk, cracker crumbs, salt, pepper, and finely chopped onion until a cohesive mixture forms.
Shape the seasoned meat mixture into a loaf and position it on a plate or shallow baking dish. Cover tightly and refrigerate overnight to allow the flavors to meld.
The following day, remove the meat loaf from the refrigerator and slice it into uniform 1/2-inch thick portions or form into patties.
Lightly coat each beef slice or patty in flour, ensuring an even dredge.
Heat a skillet over medium-high heat and brown the floured beef pieces on both sides until a crust forms.
Carefully transfer the browned beef slices to a baking dish, arranging them in a single layer.
In a separate bowl, whisk together the cream of mushroom soup with an equal measure of water or broth until fully blended and smooth.
Pour the prepared mushroom mixture over the beef slices in the baking dish, ensuring full coverage.
Place the dish in a preheated oven at 350°F and bake for 1 hour, or until heated through and the gravy has thickened to a velvety consistency.
Serve generously over mashed potatoes or egg noodles for a complete, satisfying meal.