Blend all ranch ingredients except the fresh dill and chives until completely smooth and creamy. Stir in the fresh herbs. Chill briefly if you have time. If the dressing is too thick to easily coat the salad, blend in a splash more milk or water until it’s pourable but still creamy.
Add the chopped lettuce to a large bowl. Top with chicken or turkey, red onion, tomatoes, avocado, and herbs. Season lightly with salt and pepper.
Drizzle in ¾–1 cup of the ranch dressing (start with ¾ cup). Toss thoroughly so everything is evenly coated. Add more dressing only if needed. Taste and adjust seasoning.