Step 1

Cut the celery stalks on the bias into ½-inch slices. Coarsely chop the leaves. Transfer to a large bowl. 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.

Step 2

Cover and marinate in the refrigerator for at least 1 hour or up to Toss again and serve!