Prep Your Bread: Lay the four slices of sourdough bread out on a clean work surface. Using a butter knife, spread the softened butter evenly across one side of each slice, going right to the edges.

Assemble the Base: Turn two of the slices over so the buttered side is facing down. On the unbuttered side, spread 2 tablespoons of pesto evenly over each slice.

Layer the Cheeses: Divide the cheese between the two pesto-covered slices. I find it works best to start with a layer of cheddar, followed by all the mozzarella, and then finish with the remaining cheddar. This seems to help everything melt together nicely.

Season and Top: Add a little crack of freshly ground black pepper over the cheese. Place the remaining two slices of bread on top, with the buttered side facing up.

Heat the Pan: Place a large, heavy-based non-stick frying pan or griddle over a medium-low heat. Let it warm up for a minute or two. Don't add any extra oil or butter to the pan.

Grill the First Side: Carefully place the assembled sandwiches into the warm pan. Cook for 4-5 minutes. What works best for me is to press down gently with a spatula a few times during cooking to encourage an even, golden crust.

Flip and Grill the Second Side: Once the bottom is a deep golden brown, carefully flip the sandwiches over using a wide spatula. Cook the second side for another 4-5 minutes, until it's equally golden and the cheese is completely melted and beginning to ooze from the sides.

Rest and Serve: Remove the sandwiches from the pan and let them rest on a chopping board for a minute. This allows the cheese to set slightly, making it easier to slice. Cut them in half diagonally and serve immediately.