In a large bowl combine mayonnaise, horseradish, sour cream, Parmesan, pepperoncini, pepperoncini juice, lemon juice, oregano, black pepper and red pepper flakes. (You can easily make this part a day ahead. Don't worry if the consistency is thin, the pasta will soak it all up.)
3/4 cup mayonnaise, 1/4 cup prepared horseradish, 1/4 cup sour cream, 1/2 cup shredded Parmesan cheese, 1/3 cup finely chopped pepperoncini (or use the already sliced version), 1/3 cup pepperoncini juice (from the jar) , 1 tbsp fresh lemon juice, 2 tbsps fresh oregano, minced, 1/2 tsp red pepper flakes