put potatoes in a large pot over high heat, cover with water, and season with salt. Bring to a boil, reduce heat to low, and cook until potatoes are fork tender, 15 to 20 minutes. Drain and set aside to cool.
Meanwhile, in a large bowl, stir together the mayonnaise, vinegar, oil, and mustard until well mixed. Add the onion, celery, and garlic and mix until well combined. When the potatoes are cool enough to handle, cubed them and add to the bowl. Add the thyme and parsley, season with salt and pepper, and toss. Serve immediately. Serves four.