How to Make It
Combine miso and 1 tablespoon water in a bowl, stirring with a fork until miso dissolves. Add yogurt, stirring until smooth. Cover and refrigerate until ready to use.
Cut potatoes in half (or quarters if potatoes are larger). Place in a Dutch oven 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. Place in a bowl.
Finely dice half of onion; thinly slice other half. Heat a large skillet over medium-high. Add 1 tablespoon oil to pan; swirl to coat. Add diced onion; cook 4 minutes or until softened. Slice any larger mushrooms. Add mushrooms to pan; cook 3 minutes or until tender, stirring constantly.
Add onion mixture to potatoes. Add miso mixture, salt, and pepper; toss.
Heat skillet over medium-high. Add remaining 1 tablespoon oil; swirl to coat. Add sliced onion; cook 4 minutes or until softened, stirring occasionally. Top potato salad with sautéed onion.