Heat the olive oil in a frying pan and add the butter and onion. Cook over a medium-low heat, stirring often, until the onion starts to turn golden. Add the apple slices and continue to cook until softened.
Sprinkle in the sugar and cook until the onion and apple turn a rich, golden brown. Remove from the heat and stir in the balsamic vinegar (optional).
Pre heat grill to high and toast the bread on one side only. Heap the mixture on to the untoasted side. Top with the Cheshire cheese and 2 sprigs of rosemary or thyme. Toast under the grill for a few moments until the cheese starts to melt and bubble.
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