Combine all ingredients in small bowl. Cover and refrigerate for at least 8 hours (24 hours is best).
Sandwiches
Preheat grill to high. Spray with olive oil and place mushrooms on grill. Season with salt and pepper. Cook mushrooms 5-10 minutes, or until softened. Transfer to plate and cool.
To assemble sandwiches, spread olive salad onto bottom of ciabatta bread. Top with mushrooms and cheese. Press the other half of the ciabatta loaf onto the top and use a sharp knife to cut into 4 equal segments. Serve immediately or wrap and refrigerate upside down for up to a day.
Notes
Prep time doesn't include time that salad marinates in the refrigerator.
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