In a bowl, whisk together vinegar, sugar, and salt. Add the onion and toss to combine. Let stand for 40-60 minutes, stirring occasionally.
Make the Salsa Verde
In a blender or food processor, pulse together parsley, vinegar, capers, lemon zest, anchovy fillets, and garlic cloves until finely chopped. Scrape the bowl. With the blender or food processor processor running, add olive oil in a steady stream. Season to taste with salt and pepper.
Make the Sandwiches
Layer ciabatta buns with baby arugula, sliced porchetta, and pickled onions. If desired, chop some of the crackling from the porchetta and sprinkle over the meat. Drizzle with salsa verde. Serve immediately.
Notes
*The amount of garlic you use will depend on how garlicky your porchetta is. If you're using my porchetta recipe, which infuses the meat with the garlic-herb rub, use one clove of garlic in the salsa verde. If your porchetta meat isn't as garlicky, or if you really love garlic, use two cloves.