2Bring a pot of water to a boil; cook asparagus in boiling water until bright green, 1 to 2 minutes. Drain asparagus and plunge into ice water to stop cooking. Drain and transfer asparagus to a baking dish.
3Whisk soy sauce, rice vinegar, vegetable oil, agave nectar, garlic, ginger, sesame oil, and cayenne pepper in a large bowl; pour marinade over asparagus, adding water if necessary to submerge asparagus. Cover baking dish and refrigerate for 2 hours or overnight.
4Drain marinade and sprinkle sesame seeds over asparagus before serving.