Mix together the mustard, beer or milk and cheese in a small saucepan. Over a low heat, allow the cheese to melt. Stir occasionally and don't let it boil, as the cheese will curdle. Remove the pan from the heat and leave to cool and thicken.
Heat a frying pan over a high heat with the oil. Season the fish and place it into the pan, skin-side down. Cook for 4–5 minutes until the skin is crispy, then turn the fish over and cook for a further minute on the other side.
Spread the cheese mixture over the 4 pieces of cod and place under a preheated grill. Grill until golden brown.