French Onion Green Bean Casserole

“The iconic green bean casserole is reworked with the flavors of French onion soup — caramelized onions and Gruyere cheese.” Ingredients For the onions: 2 tablespoons butter 2 large yellow onions, thinly sliced lengthwise salt and ground black pepper to taste 1 tablespoon sherry vinegar For the white sauce: 2 tablespoons butter 2 1/2 tablespoons all-purpose flour 2 1/2 cups milk 1 pinch ground nutmeg cayenne pepper to taste (optional) 1/4 teaspoon ground thyme For the topping: 2 tablespoons melted butter 2/3 cup panko bread crumbs 2 pounds fresh green beans, trimmed 4 ounces shredded Gruyere cheese, divided 1/3 cup grated Parmesan cheese Directions...