3 tablespoons olive oil
1 half of medium sweet onion thinly sliced
1 half of medium sweet onion diced
1 c 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
4 cups frozen French-cut green beans
1/3 cup reduced-fat sour cream
3 tablespoons buttermilk powder
1 teaspoon paprika
1/2 teaspoon garlic powder
Preheat oven to 400°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 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.