Butter one side of each piece of sourdough. Bring a cast iron pan to a medium heat. Place two pieces of sourdough, butter side down, into the pan. Top with a slice of cheese, red onion, chunks of smoked salmon, herbs and salt and pepper. Place another piece of cheese on top and a slice of sourdough, butter side up.

Fry for 2 - 3 minutes on each side until golden brown, use the back of a spatula to press down on sandwiches. Repeat with remaining ingredients.