Snap the tips from the asparagus and save for future use. Slice the stalks lengthwise into strips 1/2" wide, then cut these into 1 1/2" lengths. Over high heat, bring 2 c water to a boil.
Add the asparagus, return to the boil, and drain immediately in a sieve. Run under cold water to cool them quickly, and pat dry with paper towels.
In a small mixing bowl, combine the miso with the dry mustard & mix until smooth. Spread half the mixture in a shallow baking dish & cover with a double thickness of cheesecloth the size of the dish.
Place the asparagus in one layer on the cheesecloth & cover with another double thickness of cheesecloth. Top with the remaining miso & mustard mixture. Marinate for about 3 hours at room temperature, or refrigerate overnight.
Discard the dressing before serving the asparagus.