Prepare the oven: Preheat the oven to 350°F and lightly grease a 9-by-13-inch baking dish.
Mix the beef: Combine the ground beef, cracker crumbs, egg, milk, diced onion, garlic powder, parsley, Worcestershire sauce, salt, and pepper until just combined.
Shape the patties: Form the mixture into six oval patties about 1/2 inch thick, then lightly coat each one with flour.
Brown the steaks: Heat the oil in a skillet over medium-high heat and brown the patties for about 2 minutes per side. They do not need to be fully cooked.
Make the gravy: Cook the sliced onion in the skillet for 3 to 4 minutes. Whisk in the cream of mushroom soup, beef broth, and milk.
Assemble: Place the patties in the baking dish and pour the onion gravy evenly over them.
Bake: Cover with foil and bake for 35 minutes. Uncover and bake for another 10 to 15 minutes, until bubbling and fully cooked.
Rest and serve: Let the bake rest for 5 minutes, garnish with parsley, and serve with mashed potatoes or vegetables.