Green Bean Casserole

Name: Green Bean Casserole
Ingredients: 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 (see Ingredient Note); 1 teaspoon paprika; 1/2 teaspoon garlic powder