In a small bowl combine onion and vinegar, pressing onion into liquid to cover as much as possible. Cover and let stand at room temperature 30 minutes or chill up to 24 hours, stirring occasionally.Advertisement
Cook asparagus, covered, in enough boiling water to cover 2 to 3 minutes or until crisp-tender; drain. Rinse with cold water to cool quickly; drain again. In a large bowl combine asparagus, arugula, and mint.
Using a slotted spoon, transfer onion to asparagus mixture. For dressing, pour 3 Tbsp. of the vinegar from onion bowl into a small screw-top jar. Add remaining ingredients; cover and shake well. Drizzle asparagus mixture with dressing; toss gently to coat.
If you have pencil-thin asparagus spears, leave them whole, but if the spears are thicker, cut them into 2- to 3-inch lengths.