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Bring all the pineapple pickle ingredients to the boil with 100ml water. Add the pineapple chunks and toss in the pickle. Allow to cool, then transfer to a bowl, cover with cling film and leave in the fridge to marinate for at least 30 mins.
Spread the sourdough slices with mayonnaise on one side. Put half the butter and the rapeseed oil in a frying pan over a medium heat. When the butter has melted, put one slice of bread in the pan, mayo-side down, and top with a slice of cheddar, some spring onion and pickled pineapple, and more cheese and sriracha, to taste. Top with the other piece of bread, mayo-side up.
Cook for about 3 mins or until crispy and golden underneath, then turn the sandwich over and add the rest of the butter. Cook until crisp and golden on that side, and the cheddar is melting. Slice in half and serve immediately.