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
In a 4-qt. nonstick chef's pan, melt the butter over medium heat. Add shallots and sauté for 1 minute. Add mushrooms and thyme and sauté for 5 minutes. Stir in garlic and sauté for an additional minute.
Deglaze the pan with sherry and reduce the liquid until almost dry.
Whisk in milk, mustard and Parmesan cheese.
In a small bowl, combine the butter and flour to form a paste (this is called beurre manie). Whisk the beurre manie into the milk, bring to a boil and then reduce to a simmer for 5 minutes.
Add the haricot vert and season to taste with salt and pepper. Transfer everything into a casserole or baking dish. Top with Panko crumbs and Parmesan and bake for 20 minutes.
Meanwhile, heat oil in a 2.5-qt. sauce pan to 350°F/180°C. In a small bowl, combine the flour, salt and pepper. Toss leeks in the seasoned flour and shake off any excess.
Place the leeks in the hot oil and fry until golden brown. Drain on paper towels and sprinkle over the top of the baked casserole.