Rub the inside of the fondue pot with the cut side of the onion. Pour in
the ale and heat gently until bubbling. Reduce the heat and gradually stir in
the grated cheeses, then continue to stir while they melt, stirring frequently.
In a small bowl, blend the flour smoothly with the milk, then stir into
the cheese mixture with the mustard and continue to cook for a couple of minutes
until thick and creamy.