Make the dressing: Combine all the ingredients in a screw-top jar. Shake until well combined. Season as required.
Place the mushrooms in a large glass bowl. Pour two –thirds of the dressing over the mushrooms and stir to coat. Cover and stand for 30 minutes or until mushrooms absorb the dressing.
Peel and dice the avocados. Add the avocados, tomatoes, cucumber and bail to the mushrooms. Pour over the remaining dressing and toss gently to combine. Serve.