1. Combine sour cream, mayonnaise, buttermilk, vinegar, chives, mustard, sugar, 1/2 teaspoon salt, and 1/8 teaspoon pepper in medium bowl. Set aside.

2. Toss mushrooms with oil in large bowl (do not clean bowl). Grill mushrooms over hot fire until lightly charred and softened, 2 to 4 minutes per side. Return to bowl and let cool, 5 minutes. Slice mushrooms thin, then transfer to bowl with reserved sour cream mixture. Toss to combine and season with salt and pepper to taste.

Makes about 3/4 cup