Boil the potatoes in salted water for 10–15 minutes, until fork-tender. Drain and let cool slightly.
While the potatoes cook, sauté mushrooms in a little oil over medium heat until golden and tender.
In a large bowl, whisk together olive oil, mustard, maple syrup, sumac, salt, and pepper.
Cut the warm potatoes into halves or quarters (depending on size) and toss them in the dressing.
Add the sautéed mushrooms and sliced red onion. Mix gently.
Top with chopped herbs if using. Serve warm or chilled.