In a small bowl, using the back of a spoon, mash the garlic to a paste with a large pinch of salt. Add the buttermilk, mayonnaise, yogurt and vinegar and whisk until smooth. In a thin, steady stream, whisk in the oil. Stir in the scallion. Season with salt and a generous pinch each of pepper and cayenne. Applications: Use as a dipping sauce for fresh vegetables. Use as a marinade for chicken or pork. Toss with steamed vegetables, a green salad or a grape-tomato-and-red-onion salad, or with boiled or roasted potatoes.
Make Ahead: The dressing can be refrigerated in a jar for up to 5 days.