Trim off the tough ends of the asparagus, then steam until just tender and bright green.
Plunge the asparagus into cold water to halt the cooking time and drain well. Set aside.
Peel the avocados then dice the flesh.
Toss with the lemon juice and lemon zest, then add to the asparagus.
For the dressing, whisk the vinegar, balsamic vinegar, olive oil, sugar, mint and parsley.
Add salt and pepper to taste, then pour over the asparagus.
Toss lightly, refrigerate for at least 1 hour then serve.