Preheat oven to 400 degrees F. Coat a 2 1/2-quart baking dish with cooking spray.

Heat 1 tablespoon 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, onion powder, 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.

Whisk the remaining 1/3 cup flour, paprika, garlic powder and the remaining 1/4 teaspoon salt in a shallow dish. Add sliced onion; toss to coat. Heat the remaining 2 tablespoons oil in a large nonstick skillet over medium-high heat. Add the onion along with any remaining flour mixture and cook, turning once or twice, until golden and crispy, 4 to 5 minutes. Spread the onion topping over the casserole.

Bake the casserole until bubbling, about 15 minutes. Let cool for 5 minutes before serving.

Ingredient notes:

Don't use the high-sodium "cooking sherry" sold in many supermarkets. Instead, purchase dry sherry sold with other fortified wines.

Look for buttermilk powder, such as Saco Buttermilk Blend, in the baking section or with the powdered milk in most supermarkets.