Start with the base: In a medium bowl, add 1 cup Greek yogurt and 2–3 tablespoons mayonnaise.
Whisk until smooth.
Thin it out: Add 2–4 tablespoons buttermilk, a little at a time, whisking until it reaches your preferred consistency. For dip, keep it thicker. For dressing, make it pourable.
Add acid: Stir in 1 tablespoon lemon juice and 1 teaspoon apple cider vinegar.
Taste and adjust as needed.
Season: Add 1 small grated garlic clove (or 1/2 teaspoon garlic powder), 1/2 teaspoon onion powder, and 1/2 teaspoon Dijon mustard. Mix well.
Herbs time: Fold in 1 tablespoon each chopped fresh dill, chives, and parsley. If using dried, use 1 teaspoon each.
Salt and pepper: Add 1/2 teaspoon kosher salt and several grinds of black pepper.
Taste and tweak to your liking.
Chill: Cover and refrigerate for at least 30 minutes. This helps the flavors blend and the dressing thicken slightly.
Final check: Before serving, stir and adjust thickness with a splash more buttermilk or a spoonful of yogurt.