Place the Russet potato pieces into a large saucepan, and cover with salted water. Bring to a boil, turn the heat down, and simmer for 10 minutes. Add the sweet potato, and cook about 15 minutes more. Remove a piece of each potato, and cut it in half to see if it is cooked enough. Once the potatoes are tender, add corn kernels; cook another 30 seconds. Drain through a colander. Fill the saucepan with cold water, and drop vegetables into water. Cool for 5 minutes, and drain.
In a large bowl, whisk together mustard, lime juice, cilantro, and garlic. Slowly whisk in oil. Mix in salt and black pepper.
Cut cooled potatoes into 1 inch cubes, and add to dressing along with cucumber, and red onion. Toss well. Serve at room temperature or chilled. Toss the peanuts in just before serving.