Pat dry the mozzarella cheese with paper towels to prevent a soggy sandwich. Butter the outside of two slices of sourdough bread generously.

In a non-stick frying pan, place the buttered slices of sourdough bread butter-side down. Layer one slice with mozzarella cheese, spread basil pesto, add sun-dried tomatoes and bacon if using.

Top with another slice of mozzarella, cover with the second slice of sourdough bread, butter-side up.

Heat the frying pan over medium heat, cook undisturbed for about 3-4 minutes until golden brown, then flip.

Press down with a spatula, cook for another 3-4 minutes until cheese is melted and bread is golden.

Remove from the pan, let cool for a minute, then slice in half and serve immediately.