Preheat oven to 400°F. Coat a 2 1/2-quart baking dish with cooking spray.
Heat 1 tablespoon olive oil in a large saucepan over medium heat. Add diced onion and cook, stirring often, until softened and slightly translucent, about 4 minutes. Stir in mushrooms, minced garlic, 1 teaspoon salt, thyme and pepper. Cook, stirring often, until the mushroom juices are almost evaporated, 3 to 5 minutes. Sprinkle 1/3 cup flour over the vegetables; stir to coat.
Add milk and sherry and bring to a simmer, stirring often. Stir in green beans and return to a simmer. Cook, stirring, until heated through, about 1 minute. Stir in sour cream and buttermilk powder. Transfer to the prepared baking dish.
To make topping: Mix 1 cup of seasoned bread crumbs with 1.5 tablespoon of olive oil. Spread the bread crumb topping over the casserole.
Bake the casserole until bubbling, about 15 minutes. Let cool for 5 minutes before serving.