Heat 5 tablespoons of butter in a 2-quart heavy saucepan over moderate heat until foam subsides.

Whisk in flour and cook roux, whisking, until pale golden, about 2 minutes.

Gradually add milk and broth, whisking, and bring to a boil, whisking.

Reduce heat and simmer gently, whisking frequently, until veloute sauce is silky and thick, about 25 minutes.

Reserve 1/2 cup sauce for topping.

Cook shallot in remaining tablespoon butter in a 10-inch heavy skillet over moderate heat, stirring, until softened, 2 to 3 minutes.

Add mushrooms and cook, stirring, until lightly browned, about 4 minutes.

Stir into veloute sauce along with chicken and Sherry.

Cut tips from asparagus and set aside.

Cut enough stalks into 1/4-inch pieces to measure 1 cup, reserving remainder for another use.

Cook tips in a saucepan of boiling salted water until crisp-tender, 2 to 4 minutes.

Transfer to a bowl of ice and cold water with a slotted spoon.

Scoop out tips and drain on paper towels, then reserve for topping.

Cook and drain chopped asparagus in same manner and stir into veloute sauce with salt and pepper to taste.

Preheat oven to 350°F.

Spread 1/4 cup filling across center of 1 crepe and roll into a cylinder.

Transfer, seam side down, to buttered baking dish, then assemble more crepes in same manner, fitting them snugly.

Bake in middle of oven 15 minutes.

Stir together yolk, 2 tablespoons cream, and reserved 1/2 cup sauce until smooth.

Beat remaining 3 tablespoons cream with a whisk until it holds soft peaks, then fold into yolk mixture.

Spoon topping over crepes and broil 5 to 6 inches from heat until lightly browned, about 2 minutes.

Heat butter in skillet, then add reserved asparagus tips and toss until warm.

Put 2 crepes on each of 8 plates and top with asparagus tips.