Whisk 1/4 cup buttermilk, 1/4 cup mayonnaise, 1/4 cup plain Greek yogurt, 1 Tablespoon minced shallot or red onion, 1 Tablespoon minced fresh cilantro, 1 Tablespoon lemon juice, 2 teaspoons minced fresh dill, 1/2 teaspoon garlic powder, 1/4 teaspoon salt, 1/4 teaspoon black pepper, and a pinch cayenne together in a medium bowl. Refrigerate until needed. Will hold for up to 5 days in the fridge.
Notes
I use dried buttermilk (follow instructions on can), but real, fresh buttermilk will work great too. Don't try to use a soured milk substitute for salad dressing.