1. Steam the asparagus until crisp-tender, about 4 minutes. Rinse under cold running water and drain. Cut into 1/4-inch dice.
2. In a bowl, combine the onion with the mascarpone, lemon juice and zest. Fold in the asparagus and season with salt and pepper. Cover and refrigerate until chilled, at least 30 minutes and up to 3 hours.