Mortadella Sandwich with Pickled Onions and Provolone

Ingredients

• 4 squares of focaccia (10cm each), cut in half horizontally
• For spreading: softened butter
• 12 thin slices mortadella
• 8 thin slices provolone dolce
• 1 bunch rocket leaves, trimmed
• Pickled onions
• 250 ml red wine vinegar (1 cup)
• 40 gm caster sugar
• 5 thyme sprigs
• 1 fresh bay leaf
• 1 Spanish onion, thinly sliced on a mandolin