Soak the sliced onion in cold water for 10 minutes, then drain and rinse.
Bring salted water to a boil and cook the pasta until tender, about 6 to 10 minutes.
Drain the pasta and rinse under cold water to cool it down.
In a large bowl, whisk together olive oil, red wine vinegar, pepperoncini brine, oregano, and salt.
Add the cooled pasta to the dressing and mix well to coat.
Stir in the soaked onion, tomatoes, green bell pepper, Kalamata olives, feta, and optional pepperoncini.
Taste and adjust seasoning with salt and pepper as needed.
Serve immediately or refrigerate for at least 30 minutes to meld flavors.