Peel and cut potatoes in 1" cubes. Place in saucepan and cover with cold water. Bring up to a gentle simmer and cook until tender, when a knife slides in and out of cubes without resistance, about 10-15 minutes. Drain potatoes and turn out onto a baking sheet.
Whisk mustard with 2 tbsp of juice from a jar of pickles. Pour over hot potatoes and toss with fingers to coat. Spread out potatoes and refrigerate for 30 minutes until cool.
Combine pickle, celery, red onion and remaining 1 tbsp pickle juice with salt and celery seed. Add mayonnaise and sour cream and whisk together.
Add potatoes and toss. Cover with plastic wrap and refrigerate 30 minutes longer. Add diced egg and fold to combine.