Place potatoes in a large pot; cover with cold water by 1 inch and season generously with salt. Bring to a boil; reduce to a simmer and cook until potatoes are tender, about 15 minutes. Run under cold water to cool slightly, then drain.
Meanwhile, in a large bowl, whisk together oil, mustard, vinegar, shallot, parsley, and thyme; season with salt and pepper. Add potatoes and onion and toss to combine. Serve at room temperature. To store, refrigerate, up to overnight.
If you don't have sherry vinegar, try cider or champagne vinegar instead. Use your favorite fresh herbs in this potato salad to make it your own.