In a bowl, add roasted red peppers, kalamata olives, pepperoncini peppers, red onion and feta cheese. Stir to combine.
Spread 1-2 tsp butter on one side of each slice of bread.
Heat a large skillet. Place one piece of bread, butter side down, on skillet. Top with ¼ cup of mozzarella cheese and half of the feta fixture, then add another ¼ cup of mozzarella cheese. Top with another piece of bread, butter side up.
Flip sandwich in pan once the bottom piece of bread is golden brown. Cook on other side until the second side is golden brown and the cheese is melted.