The tomato water can be refrigerated overnight. Bring to room temperature before using.
Look for imported Italian buffalo mozzarella, which comes packed in water in 500-gram plastic containers, at specialty markets and at cheese shops.
The shoulders of Mount Vesuvius, near Naples, are the home of the Coda di Volpe grape—so named because the long bunches resemble the tail of a fox. It's used for minerally, dry Lacryma Christi del Vesuvio whites, perfect for this savory soup.