In a large bowl, combine the Greek yogurt, mayonnaise, lemon juice, garlic, celery seed, dry mustard powder, dried onion powder, salt, pepper, dried oregano and crushed red chile flakes. Whisk in the buttermilk, and add the dill and parsley last. Cover and refrigerate at least 1 hour to let the flavors marinate and the celery seed soften. Taste and adjust the seasoning with salt as needed before serving. Store in the fridge up to 5 days.