For the casserole:
4 Tbsp. unsalted butter
2 medium shallots, minced
4 c. assorted mushrooms, sliced
1 tsp. fresh thyme, minced
2 cloves fresh garlic, minced
1/2 c. dry sherry
2 c. whole milk
2 tsp. Dijon mustard
1/2 c. grated Parmesan cheese
2 Tbsp. unsalted butter, cold
2 Tbsp. flour
2 lbs. haricot vert, cleaned, trimmed and blanched
salt and fresh ground black pepper
4 Tbsp. panko bread crumbs
2 Tbsp. grated Parmesan cheese
For the topping:
2 leeks, white part only, finely sliced
1/4 c. flour
1 tsp. salt
1 tsp. pepper
2 c. vegetable oil