Gather all your ingredients. Chop the roasted red peppers, slice the Kalamata olives, and thinly slice the red onion and pepperoncini peppers. Crumble the feta cheese and shred the mozzarella cheese if necessary.
On one slice of sourdough bread, spread a generous layer of butter. Layer half of the mozzarella cheese, followed by the roasted red peppers, Kalamata olives, pepperoncini, red onion, and feta cheese. Top with the remaining mozzarella and another buttered slice of sourdough.
Heat a skillet or griddle over medium heat until warm.
Place the assembled sandwich on the skillet. Cook for 3-4 minutes on one side until golden brown and crispy, pressing down with a spatula.
Flip the sandwich and cook for another 3-4 minutes until both sides are golden and the cheese is melted. Adjust heat if necessary.
Remove the sandwich from the skillet and let it sit for a minute before slicing in half. Serve warm and enjoy!
