Steeping onion: Add vinegar to the bowl with the onion and let it rest for 10 minutes.Separate vinegar from onions and set both aside.
Mixing the salad: In a large mixing bowl, combine potato, eggs, carrot, green beans, onion, and stir to mix thoroughly. Mix in vinegar, oil, and mustard. Add the apple and combine. Season with salt and pepper to taste. If you won't serve right away, cover and keep chilled for later.
Serving: This salad keeps refrigerated for about a day. Once chilled, serve right out of the fridge so the eggs don't get spoiled.
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Notes
The potatoes and carrots should be boiled beforehand with a teaspoon of salt added to the water, or you can use frozen mixed veggies (thawed before starting) for a quick version.Peel and dice the apple right before adding it to the salad, otherwise, it will turn black.Served as a light meal, this serves 4, as a salad, serves 6.