Place evaporated milk in a pot with cream, onion, mustard powder and pepper.
Heat until almost boiling. Remove from heat, add cheese and soup powder and stir until cheese has melted and sauce has thickened. Allow to cool before using (it will thicken further as it cools). This mixture can be made in advance and stored in a container in the fridge for up to a week
To cook rolls, preheat oven to 180˚C and line 2 oven trays with baking paper.
Spread one side of each slice of bread with butter or mayonnaise and remove crusts if desired (traditionally they aren’t removed).
Spread 2 tbsp of the cooled cheese mixture on the unbuttered side of each slice, and then roll up firmly, placing joined-edge down on the prepared tray.
Bake until crisp and golden (15-20 minutes). Allow to cool for a few minutes before serving