Preheat the oven to 450 degrees . In a large bowl, combine the ground beef, onion, breadcrumbs, egg, milk, 1/2 teaspoon each salt and pepper and the garlic powder. Mix gently until just combined.
Using greased hands, divide the meat mixture into 8 equal portions and shape each portion to resemble a frankfurter. Transfer to a baking sheet and bake until browned and cooked through, 15 to 20 minutes. Serve immediately in the hot dog buns along with the mustard, ketchup, relish and sliced onions.