Mix ground beef, milk, crackers, 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 the 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 in the hot skillet until browned, 2 to 3 minutes per side. The inside 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).