3 tablespoons canola oil, divided
1 medium sweet onion (half diced, half thinly sliced), divided
8 ounces mushrooms, chopped
1 tablespoon onion powder
1 1/4 teaspoons salt, divided
1/2 teaspoon dried thyme
1/2 teaspoon freshly ground pepper
2/3 cup all-purpose flour, divided
1 cup low-fat milk
3 tablespoons dry sherry (see Ingredient Note)
1 pound frozen French-cut green beans (about 4 cups)
1/3 cup reduced-fat sour cream
3 tablespoons buttermilk powder
1 teaspoon paprika
1/2 teaspoon garlic powder
Preheat oven to 400°. 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.