Place slices of veal between 2 pieces of plastic wrap and, using a meat mallet, pound to 1mm-thick.
Lay veal on a clean work surface, place a slice of mozzarella in the middle and top with 2 basil leaves. Fold veal like an envelope to form a 10cm square, enclosing filling.
Dust veal wallets in seasoned flour. Heat oil and butter in a large heavy-based frying pan over medium-high heat, add veal wallets, and fry each side until golden (2-3 minutes). Transfer to a warm plate and keep warm. Deglaze pan with red wine vinegar, stir in sugar to dissolve. Remove pan from heat and stir in tomato. Season to taste with sea salt and freshly ground black pepper. Serve veal wallets with tomato spooned over.
Note Girello is also sold as veal nut and may need to be ordered ahead from your butcher.