In the bowl of a food processor, combine mayonnaise, the drained pepperoncini, pepperoncini juice, capers, parsley, and garlic. Pulse the ingredients until they are roughly chopped and combined. For best results, refrigerate the aioli for at least 30 minutes before serving to allow the flavors to meld. Enjoy as a flavorful dip, spread, or sauce for sandwiches, burgers, or grilled vegetables. Serves 4-6.