1 1/2poundstoasted ciabattasoft breadsticks, or garlic bread, for serving
Heat the oven to 375°F and arrange a rack in the middle.
Place half of the mozzarella, half of the pepperoni, the cream cheese, ricotta, Parmesan, sun-dried tomatoes, salt, oregano, and red pepper flakes in a food processor fitted with the blade attachment. Process until smooth, about 45 seconds (there may still be a few small bits of pepperoni).
Transfer the mixture to a 9-inch pie plate and smooth the top. Evenly sprinkle with the remaining mozzarella and pepperoni.
Bake until the dip is heated through and bubbling around the edges and the cheese on top is melted, about 15 minutes. Serve immediately with the ciabatta bread, breadsticks, or garlic bread for dipping.