Peel and cut the sweet potato into quarters. Place in a pot of salted water to boil for about 20 minutes. Once the potatoes are tender, add corn kernels; cook another 30 seconds. Drain through a colander.
Cut the the cucumber lengthwise into quarters then slice. Set aside.
In a separate bowl whisk the mustard, fresh lime juice, chadon beni, garlic, salt and black pepper. Slowly whisk in the oil.
Cut cooled potatoes into 1 inch cubes. Add the dressing, corn, cucumber, and onion to the sweet potato. Toss well.
Serve at room temperature or chilled. Sprinkle with peanuts just before serving.