Green Bean Casserole

6 T unsalted butter, plus more for dish
1 med onion
1 lb mushrooms, quartered
2 t coarse salt
1/2 t freshly ground pepper
24 oz bag Trader Joe's Frozen French Green Beans
6 T all-purpose flour
2 C milk
8 oz Trader Joe's Groumet Fried Onion Pieces

1. In a large skillet over medium heat, melt 2 T butter. Add onion, and saute until it begins to soften, about 4 minutes. Add mushrooms, and cook until softened, about 8 minutes. Season with 1 t salt and 1/4 t pepper. Set aside to cool.

2. Cook Trader Joe's Frozen French Green Beans according to cooking instructions on bag.

3. Melt the remaining 4 T butter in a medium saucepan over medium-low heat. Add 1/4 C 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 t salt and 1/4 t pepper. Remove from heat, and let cool to room temperature, stirring occasionally. Pour over beans, and toss to combine.

4. Butter a 9" x 13" glass or ceramic baking pan and spread the green bean mixture.

5. 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 Trader Joe's Gourmet Fried Onion Pieces over top and serve.