Classic Green Bean Casserole

Name: Classic Green Bean Casserole
Ingredients: 1 large red onion, thinly sliced lengthwise; 1 cup buttermilk; Vegetable oil, for frying; 1 cup all-purpose flour; 1 cup panko breadcrumbs; 2 tablespoons finely chopped fresh parsley; Kosher salt; 2 pounds green beans, trimmed and halved; 3 tablespoons unsalted butter; 1 pound white button mushrooms, quartered; 2 cloves garlic, minced; 2 teaspoons fresh thyme; 2 tablespoons all-purpose flour; 1 1/2 cups half-and-half; 1 1/2 cups low-sodium chicken broth; Pinch of cayenne pepper; 1/2 cup grated parmesan cheese (about 1 ounce)