Alfred Portale makes this vinaigrette to dress his pear salad at Gotham Bar and Grill in New York City.
In a medium bowl, whisk together vinegar and mustard. Gradually whisk in the walnut oil until emulsified. Season with salt and pepper. Cover, and refrigerate until ready to use. Before using, whisk well. Store in an airtight container, refrigerated, up to 1 day.