Bring onion, carrot, and wine to a boil in a
medium saucepan. Add mussels and return to a boil.
Remove from heat, cover, and let sit 10 minutes.
Meanwhile, cook asparagus in a large pot of
boiling salted water until bright green and crisp-
tender, about 5 minutes. Using tongs, transfer to
a bowl of ice water. Let cool, then drain and set
aside. Reserve pot with water.
Strain mussel mixture through a sieve into another
medium saucepan (you should have about 1/2 cup).
Discard (or eat!) mussels and vegetables. Bring
cooking liquid to a boil, reduce heat, and simmer
until reduced to 1/4 cup.
Whisking constantly, add butter, a piece at a
time, incorporating before adding the next piece.
Whisk until sauce is emulsified. Whisk in vinegar;
season with salt.
Just before serving, warm asparagus in reserved
pot of hot water. Drain, pat dry, and transfer to
a platter. Gently mix locust flowers, if using,
into sauce and spoon over asparagus.
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