Preheat oven to 350° F.
Cook green beans in boiling salted water to cover in a Dutch oven 4 to 6 minutes or to desired degree of doneness; drain.
Plunge into ice water to stop the cooking process; drain and pat dry.
Melt 2 Tbsp. butter in Dutch oven over medium heat; whisk in flour until smooth. Cook, whisking constantly, 1 minute.
Gradually whisk in 1 1/2 cups milk; cook, whisking constantly, 3 to 4 minutes or until sauce is thickened and bubbly.
Remove from heat, and whisk in buttermilk and next 4 ingredients.
Melt 1 tsp. butter in a medium skillet over medium-high heat; add mushrooms, and sauté 6 to 8 minutes or until lightly browned.
Remove from heat; let stand 5 minutes.
Gently toss mushrooms and green beans in buttermilk sauce.
Place in a 13- x 9-inch or 3-qt. baking dish coated with cooking spray.
Combine French fried onions and next 2 ingredients; sprinkle over green bean mixture.
Bake at 350° for 25 to 30 minutes or until golden brown and bubbly.
Serve immediately.
