Mushroom-Potato Salad with Miso "Mayo"

This is a terrific umami-rich potato salad especially loved by the editors of Cooking Light mag. They said it was one of their favorite recipes ever.  Hope it is yours, too. It holds up well and can be made a day ahead -- but bring to room temperature before serving. 

1 tablespoon white (shiro) miso
1/2 cup plain low-fat yogurt (not Greek-style)
1-1/2 pounds small new potatoes
1 large red onion
2 tablespoons olive oil
5 ounces exotic mushroom blend (or just shiitakes), sliced

Combine miso and 1 tablespoon water in a small bowl, stirring with a fork until miso dissolves.  Whisk in yogurt and stir until smooth.  Cover and refrigerate until ready to use.  Cut potatoes in half (or quarters if they are larger).  Place in a small pot with water to cover; bring to a boil over high heat.  Reduce heat to medium; cook 15 minutes or until tender (but not falling apart). Drain; rinse with cold water and pat dry.  Place in a bowl.  Finely dice half of an onion to get 1/2 cup.  Heat 1 tablespoon oil in a large skillet.  Add diced onion and cook 4 minutes until softened.  Add sliced mushrooms.  Cook 3 minutes or until tender, stirring constantly.  Add mushroom mixture to potatoes.  Add miso mixture, salt and pepper to taste.  Toss well. 

Heat skillet over medium-high heat.  Add remaining tablespoon oil.  Thinly slice remaining onion and cook 4 minutes until soft, stirring occasionally.  Top potato salad with sauteed onion. (Note: you can also add a bit of finely minced raw onion).  Serves 6