Preheat oven to 350 degrees.
Cook green beans in boiling water until they are done; drain.
Plunge in ice water to stop the cooking process; Drain and pat dry.
Melt 2 tbsp. butter in large pot.
Whisk in flour until smooth.
Cook, whisking constantly, 1 minute.
Gradually whisk in 1-1/2 cups milk; whisking constantly 3 or 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 med. high heat; add mushrooms and saute' 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 13x9 inch or 3 quart baking dish coated with cooking spray.
Combine French's fried onions and next 2 ingredients; sprinkle over green bean mixture.
Bake at 350 degrees for 25 to 30 minutes or until golden brown!