Blitz everything in a blender or food processor until smooth.
For each sandwich:
Butter two slices of bread on one side. Spread the pesto on the opposite side of one slice bread, add 1-2 slices of cheese, then top with the other slice of bread. Fry on the stove over medium heat for about 3 minutes a side, or until golden brown.
Leftover pesto can be used as a dip for veggies, a pasta sauce, or mixed into scrambled eggs. When storing in the fridge, add a thin layer of olive oil to the top to help prevent an unattractive browning of the pesto. If you forget, you can just scrape the darker portions off.