Asparagus with Mussel Beurre Blanc

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.