Put all the ingredients for the jam and 60ml water in a medium saucepan, place on a medium-high heat and cook, stirring often, for 15 minutes, until the rhubarb breaks down to a jammy consistency.
Leave to cool, discard the tea bag, then spoon into a sterilised jar and store in the fridge.
For the toasties, set a large frying pan over a medium-high heat. Slather both sides of each slice of sourdough with a medium-thin layer of mayonnaise, spread a generous amount of the rhubarb jam on one side of each slice, top that with the cheese and cover with a second slice of bread.
Brush a little olive oil over the surface of the hot pan, then lay in the toasties and press down on them with a spatula, so they colour evenly. Cook for about three minutes.
Turn and cook for another three minutes on the other side, until the outside of the bread is golden brown and crisp and the cheese has melted (if the bread starts to catch, turn down the heat a little). Serve immediately, but be careful with the hot melted cheese.