In a medium bowl, stir together the roasted red peppers, Kalamata olives, sliced pepperoncini peppers, thinly sliced red onion, and crumbled feta until evenly combined.

Divide that mixture into two equal portions and set aside.

Spread softened butter on one side of each slice of sourdough bread.

Heat a large skillet over medium heat.

Place one slice of bread butter-side down in the skillet. Sprinkle 1/3 cup of the shredded mozzarella evenly over the bread, spread one of the reserved portions of the pepper-feta mixture on top of the cheese, then sprinkle another 1/3 cup of mozzarella over the mixture. Top with a second slice of bread, butter-side up.

Cook until the bottom slice is golden brown and the cheese begins to melt, about 3–4 minutes, then carefully flip the sandwich and cook until the second side is golden and the cheese is fully melted, another 3–4 minutes. Press gently with a spatula as needed.

Remove the sandwich from the skillet and let it rest for 1 minute before slicing.

Repeat steps 5–7 to make the second sandwich.