1.In a small glass bowl, combine mayonnaise, Greek yogurt, Dijon mustard, red wine vinegar, garlic powder, and oregano. Stir to combine thoroughly. Season with salt and pepper, to taste. Set aside.
2.In a larger non-reactive bowl, combine chicken breast, celery, pecans, grapes, parsley, and green onion. Add dressing and gently stir to combine all ingredients thoroughly. Season with additional salt and pepper, if desired.
3.Cover and place in refrigerator for at least 30 minutes to overnight to allow the flavors to combine. To serve, divide the chicken salad between the Romaine lettuce leaves. Garnish with parsley and/or green onions, if desired, and serve immediately.