Bring a pot of lightly salted water to a boil. Add asparagus and cook until tender, about 4 minutes. Drain in a colander and immediately immerse in ice water until cold to stop the cooking process. Drain.

Boil balsamic vinegar in a small saucepan until reduced to 1/3 the original amount, about 5 minutes. Pour reduced vinegar into a bowl and whisk olive oil into vinegar; set aside until cool, about 10 minutes. Add asparagus, onion, tomatoes, and olives to vinegar mixture and toss to coat.