1. Toast the bread lightly on both sides.
2. Butter one side of each slice.
3. Grate the cheese finely.
4. Put the cheese, milk and fat into a saucepan, and cook over gentle heat, stirring all the time, until the cheese has melted and the mixture is well blended and creamy.
5. Mix in the vinegar and mustard, and season to taste.
6. Add the gherkin. Pile the mixture on the buttered side of each slice of toast.
7. Serve as it is, or grill briefly, using moderate heat, to brown the surface.