Grate cheese and dice onion as finely as possible. In a medium mixing bowl add cheese, onion, mayonnaise and a good pinch of black pepper. Mix well then taste test and adjust seasoning as necessary.
Butter one side of each slice of bread. Divide filling among 4 slices of bread and spread the filling all the way to the edges. Top with the second slice of buttered bread. Cut in half and enjoy!
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