French Onion Green Bean Casserole

Servings: 8+
Cook Time: 30 minutes
Prep Time: 10-15 minutes

Ingredients:
For the onions:
2 tbsp butter
2 or 3 yellow onions, sliced
salt and pepper to taste
1 tbsp sherry vinegar (stirred into the onions after they are caramelized)

For the white sauce:
2 tbsp butter
2 1/2 tbsp flour
2 1/2 cups milk
pinch of nutmeg
cayenne to taste
1/4 tsp dried thyme
salt and pepper to taste

For the topping:
2/3 cup panko or other style bread crumbs
2 tbsp melted butter

For the rest:
2 lbs green beans, trimmed, blanched
salt and pepper to taste
4 oz grated Gruyere cheese, divided
1/3 cup grated Parmesan cheese