4squares of focaccia (10cm each), cut in half horizontally For spreading:softened butter12thin slices mortadella8thin slices provolone dolce1 bunchrocket leaves, trimmedPickled onions250 ml (1 cup)red wine vinegar40 gmcaster sugar5thyme sprigs1fresh bay leaf1Spanish onion, thinly sliced on a mandolin
For pickled onions, combine vinegar, sugar, thyme, bay leaf and 100ml water in a saucepan over medium heat. Stir to dissolve sugar, then transfer to a bowl and set aside to cool slightly (5 minutes). Add onion and set aside to pickle (15 minutes).
Heat a char-grill pan over high heat, grill focaccia cut-side down in batches until golden (1-2 minutes), then spread with butter. Grill mortadella, turning once, until golden (1-2 minutes). Divide among focaccia bases, top with provolone, then pickled onions and rocket, sandwich with focaccia tops and serve.