
1. Remove the celery leaves and cut the stalks on the bias into ¼” slices. Transfer to a large bowl.
2. Add the onion, garlic, vinegar, oil, capers, oregano, parsley, salt, black pepper, and red pepper flakes. Toss to combine. Taste and adjust the seasonings as needed.
3. Cover and marinate in the refrigerator for at least an hour to let the flavors meld. Toss again and serve!