Cook green beans according to directions on bag.
Melt the remaining 4 Tbl butter in a medium sauce pan over med-low heat. Add 1/4 cup flour, whisk constantly until mixture begins to turn golden, about 2 minutes. Pour in milk and continue whisking until mixture has thickened, about 3 minutes. Stir in remaining tsp salt and 1/2 tsp pepper. Remove from heat and let cool to room temperature, stirring occasionally. Pour over beans and toss to combine.
Butter a 9x13 glass or ceramic baking pan and spread the green bean mixture. Heat the broiler, positioning rack about 8 inches from heat. Cook casserole, covered, until mixture is bubbly and heated through, about 10 minutes. Uncover and cook until top is golden brown, about 30 seconds. Sprinkle fried onion pieces over top and serve.
at 9:54 AM on Nov. 25, 2010