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In a large skillet over medium heat, melt 2 Tbsp butter. Add onion and saute till it begins to soften, about 4 minutes. Add mushrooms, and cook until softened, about 8 minutes. Season with 1 tsp salt and 1/3 tsp pepper. Set aside to cool
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Cook frozen green beans according to cooking directions on bag.
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Melt the remaining 4 Tbsp butter in a medium saucepan over medium-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 1/4 tsp pepper. Remove from heat and let cool to room temperature, stirring occasionally. pour over beans, and toss to combine.
- Butter a 9"x13" glass or ceramic baking dish and spread the green bean mixture in dish.
- Heat 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 with fried onions, and serve.
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