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While the beef rests, preheat an oven to 180°C/gas mark 4 and gently transfer the cooled onions to a baking tray. Cover the tops with grated cheese, then bake in the oven until it melts and slightly colours. Sprinkle over the Panko breadcrumbs, thyme and seasoning, then return to the oven and bake until the breadcrumbs turn golden