Place fillets on a lightly greased rack in a broiler pan.
Broil 5 inches from heat (with electric oven door partially open) 10 to 15 minutes or until fish flakes easily with a fork. Flake fillets in a large bowl; set aside.
Melt butter in a heavy saucepan over low heat; add flour, stirring until smooth. Cook, whisking constantly, 1 minute; gradually add milk, and cook over medium heat, whisking constantly, 8 minutes or until mixture is thickened and bubbly.Stir in salt, pepper, and mustard. Pour over fish.
Stir in 1/2 cup breadcrumbs and next 3 ingredients; cover and chill at least 1 hour. Shape mixture by 1/4 cupfuls into patties, and coat with remaining 1 1/2 cups breadcrumbs.
Melt 1 tablespoon butter with 1 tablespoon oil in a large nonstick skillet; fry patties in batches 3 minutes on each side or until lightly browned, adding oil and butter as needed. Serve with tartar sauce.