Preheat oven to 350 degrees F. Boil fresh green beans in salted water covered for about 5 minutes. Drain and blanch on ice to stop cooking process then pat dry. Set aside.
In a saucepan over medium heat, melt 2 tablespoons of butter and whisk in flour until smooth, about 1 minute. Gradually add milk, whisking constantly for about 3 to 4 minutes until sauce is thickened and bubbly.
Remove from heat and whisk in buttermilk, ranch dressing, thyme and salt and pepper.
In a medium skillet over medium-high heat, melt one teaspoon of butter and add mushrooms. Saute for about 7 minutes, or until lightly browned, and remove from heat. Allow to cool for about 5 minutes then toss with green beans in the ranch-buttermilk sauce.
Place mixture in a 13 x 9-inch baking dish coated with spray. Sprinkle with crumbled French onions and panko over green beans and bake in the oven for 25 to 30 minutes. Top should be golden brown and bubbly.