Prepare Wish-Bone® Italian Dressing & Seasoning Mix according to package directions using extra virgin olive oil. Reserve 1/4 cup prepared Dressing & Seasoning Mix; refrigerate remaining for another use.
Pour reserved prepared 1/4 cup Dressing & Seasoning Mix over mushrooms and onion in large shallow nonaluminum baking dish or resealable plastic bag. Cover, or close bag, and marinate in refrigerator at least 30 minutes.
Remove mushrooms and onion from marinade, discarding marinade. Grill or broil, turning once, until tender, about 10 minutes . Top mushrooms (stem-side-up) with tomatoes, onion, then cheese. Grill or broil until cheese is melted. Serve on rolls, topped with baby arugula.