Preheat the oven to 180C/350F/Gas 4.
Brush the filo sheets with melted butter and then layer into a small tart tin on a baking sheet. Place in the oven and bake for five minutes, or until crisp and golden. Remove from the oven and set aside.
Heat the olive oil in a frying pan, add the onion and fry for four minutes, then spoon into the prepared flan. Top with the parma ham, gorgonzola and basil and bake in the oven for six minutes, or until the cheese has melted.
Slide onto a plate and serve.