In a small saucepan, melt butter over medium heat. Stir in flour until smooth; gradually whisk in milk. Bring to a boil, stirring constantly; cook and stir until thickened, 2-3 minutes. Remove from heat. Transfer to a small bowl; cool., Stir in tuna, 1/3 cup bread crumbs, green onion, lemon juice, salt and pepper. Refrigerate, covered, at least 30 minutes., Place remaining 1/2 cup bread crumbs in a shallow bowl. Drop 1/3 cup tuna mixture into crumbs. Gently coat and shape into a 1/2-in.-thick patty. Repeat. In a large skillet, heat oil over medium heat. Add tuna patties in batches; cook until golden brown, 2-3 minutes on each side. Drain on paper towels., Freeze option: Freeze cooled tuna patties in freezer containers, separating layers with waxed paper. To use, reheat tuna patties on a baking sheet in a preheated 325° oven until heated through.