Recipe adapted from Aran Goyoaga and her blog, Cannelle et Vanille.
In a medium pot, bring potatoes to a boil in salted water over high. Reduce heat; cook at a rapid simmer until potatoes are tender when pierced with a knife, 15 minutes. Drain, run under cool water to stop the cooking, then cut in half.
Meanwhile, in another medium pot, bring 4 cups water to a boil over high. Reduce heat to medium, carefully add eggs, and cook 7 minutes (whites will be set and yolks will be runny). Prepare a large bowl of ice water. Transfer eggs to ice water and let cool 5 minutes. Carefully peel eggs.
Arrange watercress on a large platter with potatoes, tuna, avocados, and scallions. In a small bowl, whisk together oil, vinegar, and mustard; season with salt and pepper. To serve, halve eggs and add to salad; drizzle dressing over salad.