Place mushrooms into a large bowl. Combine ovinegar, mustard, sugar and salt and pepper in a screw-top jar. Shake well to combine.
Pour dressing over mushrooms and toss well to coat mushrooms in mixture. Cover and set aside to marinate for 30 minutes, stirring every 10 minutes.
Meanwhile, grill the eggplant if necessary.
Just before serving, add olives, semi-dried tomatoes, eggplant and basil leaves to mushrooms and gently toss to combine. Season with salt and pepper and serve.