Wipe the mushrooms with a damp cloth. If they are large, quarter them; otherwise, halve them. Finely chop the scallions.
Put the mushrooms and scallions in a bowl. Add all the remaining ingredients and toss to mix. Leave the mushrooms to marinate in their dressing for 5 minutes, then toss again and set aside another 5 minutes.
Toss the mushrooms once more before serving, garnished with scallion shreds if you like.
Recipe courtesy of Madhur Jaffrey's Step-by-Step Cooking (Ecco Press, 2001)