Tip:For a dressier presentation, make the potato salad with only the scallions, olives, and feta. Then dress the beans, artichokes, and peppers individually (you’ll need a double batch of dressing). Line each plate with a few leaves of bibb lettuce drizzled with dressing, and arrange all the elements artfully on top. If you like, add a bit of canned tuna or cooked chicken.
Combine the vinegar and oregano in a small bowl; season with salt and pepper. Whisk in the olive oil and taste for seasoning.
Bring a large saucepan of well-salted water to a boil over high heat. Add the beans and boil until just tender, about 4 min. Using a strainer, scoop out the beans and rinse under cold water.
Let the water in the saucepan come back to a boil. Add the potatoes and simmer, adjusting the heat as necessary, until the potatoes are tender, 10 to 15 min. Drain and transfer the potatoes to a large bowl; add the white parts of the scallions, drizzle on a few teaspoons of the dressing, and toss to coat. Season with salt and pepper to taste. Set aside to cool slightly.
When ready to serve, add the green parts of the scallions, the green beans, artichokes, red peppers, olives, and goat cheese to the potatoes. Pour over the remaining dressing and toss to combine. Season to taste with salt and pepper and serve.