To make the relish, put the vinegar, sugar and mustard seeds in a small pan and simmer until the sugar has dissolved. Add the spring onions and cook for 1-2 minutes, then add the tomatoes. Simmer until jammy, then season and set aside to cool.
Slice the rolls in half and spread the bottom halves with a little mayonnaise.
Divide the cheese between the rolls, top each one with a spoonful of relish, and then sandwich with the top halves.