Mix ground beef, milk, saltine cracker crumbs, onion, salt, and black pepper in a bowl. Shape mixture into a narrow loaf and place onto a baking sheet; cover and refrigerate 8 hours to overnight.
Preheat oven to 350 degrees F (175 degrees C).
Heat a large skillet over medium-high heat. Cut loaf into 1-inch-thick slices and sear the 'steaks' in the hot skillet until browned, 2 to 3 minutes per side. Insides will still be pink. Arrange slices in a roasting pan. Spread mushroom soup over steaks.
Bake steaks in the preheated oven until mushroom soup is bubbling and meat is no longer pink inside, about 1 hour. An instant-read meat thermometer inserted into the center of a meat slice should read at least 160 degrees F (70 degrees C).