-Balsamic onions add a tasty tang to the traditional steak sandwich. Peel and slice two red onions, then slowly fry them in a little olive oil over a low heat, stirring occasionally, for about 30 minutes until softened and starting to turn golden. Add a couple of tablespoons of brown sugar and a tablespoon of balsamic vinegar, then cook for another 10 minutes until the mixture caramelises and thickens. -Short of time? Quickly fry brown onions with chopped garlic and chilli for an alternative onion filling. -Remember, fried egg, beetroot and thinly sliced aged cheddar also make great steak sandwich additions.