In a pot, cover potatoes with 2 inches of water. Season with salt — 1 teaspoon for every quart of water. Bring water to a boil then reduce to a low simmer. Cook 15 to 20 minutes or until potatoes can easily be pierced with a fork.While potatoes cook, set up an ice bath. Add cold water to a medium bowl then add ice. Transfer potatoes to ice bath. Then, slice potatoes into 1/2-inch rounds.Bring water that potatoes were cooked in to a boil then add green beans. Cook 2 to 3 minutes or until bright green and crisp-tender. Set up a second ice bath. Transfer green beans to ice bath.Combine potatoes, green beans, olives, spring onion and the parsley in a large bowl.Whisk vinegar, olive oil, mustard and the lemon juice until creamy. Add the feta cheese, stir well then season with salt and pepper. Toss salad with dressing to coat.Leave at room temperature 10 to 15 minutes. Toss again, season with salt and pepper (as needed) then serve.