4 large slices white rustic bread (1/2 inch thick)
1 stick butter
2 rounded tbsp. flour
About 2 cups whole milk
Salt and white pepper or finely ground black pepper
A little freshly grated nutmeg
1 tbsp. Dijon mustard
1 stalk celery with leafy top, finely chopped
1 large shallot, finely chopped
2 cloves garlic, finely chopped or grated
A handful of fresh flat-leaf parsley leaves, finely chopped
2 scant tbsp. fresh thyme leaves, chopped
1 tbsp. lemon zest (from 2 lemons)
½ cup sherry or dry (white) vermouth
1 lb. lump crabmeat, picked over, or 1 lb. king crab leg meat (from 2 lb. of legs), shredded into small pieces
2 tsp. Old Bay Seasoning
A few dashes of hot sauce (optional)
A few dashes of Worcestershire sauce
8 thin slices French ham or prosciutto cotto
About 1 1/4 cups grated Gruyère (5 to 6 oz.)
½ cup grated Parmigiano-Reggiano
4 large eggs plus a tab of butter, if making croque madames
Chopped fresh chives or tarragon, for garnish
Preheat the oven to 375°. Place the bread directly on a rack in or near the center of the oven and bake until lightly toasted, 5 to 6 minutes.
Meanwhile, in a medium saucepan or skillet, melt about 2 1/2 tbsp. butter. Whisk in the flour, then the milk; season with salt, white pepper, and nutmeg. Bring to a boil. Reduce heat to medium and gently simmer the béchamel sauce, stirring often, until thick enough to coat a spoon, about 2 minutes. Remove the sauce from heat. Whisk in the Dijon.
Brush the toast with some butter (about 2 tbsp. total) and place on a baking sheet lined with foil. Switch on the broiler.
In a large skillet, melt the remaining 3 1/2 tbsp. butter over medium to medium-high heat. Add the celery, shallot, and garlic; stir until beginning to soften, about 2 minutes. Stir in the parsley, thyme, and lemon zest; cook, stirring often, until well incorporated, about 1 minute. Add the sherry; cook until absorbed, 2 to 3 minutes. Scrape the vegetables into a bowl. Mix in the crabmeat, Old Bay, hot sauce (if using), and Worcestershire. Stir in about 2/3 cup of the béchamel sauce.
Loosely layer 2 slices of ham on top of each toast. Top with the crabmeat, dividing equally, then top with the remaining béchamel sauce, the Gruyère, and Parm. Broil until the cheeses are golden in spots, 5 to 6 minutes.
If you’d like to make croque madames, heat a tab of butter in a nonstick skillet over medium. Crack in the eggs and cook until the whites are set but the yolks are still runny, 2 to 3 minutes. Place 1 egg on top of each sandwich.
Garnish the sandwiches with chives or tarragon.