Place potatoes in a Dutch oven; add water to cover. Cut onion in half crosswise; add one half to saucepan. Bring to a boil. Add lemon and salt to cooking water. Reduce heat; cook, uncovered, until potatoes tender, 5-6 minutes., Meanwhile, chop remaining onion. Combine with eggs, mayonnaise, dill pickle relish, mustard and Cajun seasoning., Drain potatoes; rinse under cold water. Discard onion and lemon. Add potatoes to egg mixture; gently toss until well mixed (do not overmix, or potatoes will break down). Refrigerate, covered, 1-2 hours. Just before serving, sprinkle with parsley and paprika.