1 In A Large Bowl Or Baking Dish Combine Tomatoes, Onion, And Vinegar And Season With Pepper And Salt. Marinate 30 Minutes. 2 Layer Mozzarella, Tomatoes, Onions And Arugula On The Focaccia Half. Spread Some Mayonnaise On The Top Half Of The Focaccia If Desired. Press Top Half Over The Bottom, Hold Together With A Couple Of Tooth Picks.