Scrub potatoes and trim imperfections. Cut into 1/2 to 1-inch pieces. Place potatoes into a pot, cover with cold salted water, and bring to a boil. Reduce heat to medium-low, simmer for about 10-15 minutes, or until fork-tender. Drain and let cool.
In a large mixing bowl, gently toss the cooled potatoes, diced celery, chopped egg, and onion.
In a small bowl, whisk together 3/4 cup of mayonnaise, sour cream, dill, dill relish, salt, and pepper.
Pour mayonnaise mixture over potato mixture, gently toss to combined. Add more mayonnaise, if needed. Cover and refrigerate until ready to serve.