Preheat the oven to 425°F. Line a baking sheet with parchment paper.
Arrange quartered potatoes on the baking sheet in a single layer and roast for 35-40 minutes until golden and tender inside.
While potatoes roast, combine green onions, cucumber, pistachios, dill and parsley in a large bowl.
In a separate bowl, whisk together mayo, mustard, garlic, lemon juice, maple syrup, salt, pepper and chili flakes if using.
Let roasted potatoes cool for a few minutes, then toss with the dressing.
Add the potato mixture to the bowl with vegetables and herbs, then gently fold everything together.
Serve warm immediately or refrigerate for a chilled version.