Pre-heat an electric griddle to 350 or heat a sturdy skillet over medium high heat. A greased panini press would also work well here. Form the grilled cheese sandwich by smearing a tablespoon of pesto onto each slice of focaccia. Layer the prosciutto on top of the pesto on one slice, then the mozzarella next. Close the sandwich with the other slice, pesto side down.

Melt the butter either on the griddle or in the skillet and put the sandwich on it to brown on the first side for about 2 minutes. Press it down firmly while it browns to help it all melt together. Then flip it over to do the same on the second side, making sure the mozzarella completely gets melted.

Plate the sandwich and cut it in half. Serve immediately for an amazing and easy lunch or dinner with salad or soup!