Thaw fish, if frozen. Preheat oven to 450 degrees F. Lightly coat a baking sheet with cooking spray; set aside. Rinse fish; pat dry with paper towels. Cut fish into 3x1-inch strips.
Pour the 2/3 cup milk into a shallow dish. Place flour in another shallow dish. In a third shallow dish stir together bread crumbs, 2 tablespoons of the salad dressing mix, 1/4 teaspoon of the dry mustard, and the pepper. Stir in melted butter until combined (if necessary, use your fingers to break up any clumps).
Dip fish in the 2/3 cup milk; lightly coat with flour. Dip again in the milk; lightly coat with bread crumb mixture. Place fish on the prepared baking sheet. Bake for 10 to 12 minutes or until fish flakes easily when tested with a fork.
Meanwhile, for dipping sauce, in a small bowl combine sour cream, mayonnaise dressing, 2 tablespoons milk, the remaining salad dressing mix, and the remaining 1/4 teaspoon dry mustard. Add additional milk, if necessary, to reach desired consistency. Serve fish with dipping sauce.