- 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.