Use the dicing blade on a food processor to dice the potatoes, or cut them into similar-sized cubes by hand.
Place the potatoes in a pot of lightly salted water and boil for about 5 minutes, or until fork-tender but not falling apart.
Drain the potatoes and rinse with cool water to stop the cooking, then set them aside to cool.
With a clean bowl under the food processor chute, slice the Napa cabbage on the thinnest setting (or thicker if preferred).
Grate or process the fennel and red onion using the coarse grating blade, then toss them with the sliced cabbage in the bowl.
Add the chopped parsley and drained capers to the cabbage mixture and toss to combine.
Make the dressing by whisking together mayonnaise, sour cream, reserved caper juice, Dijon mustard, sugar, cayenne, and salt and pepper to taste until smooth.
Toss the cooled potatoes and the dressing with the slaw mixture until evenly coated.
Adjust seasoning as needed and top with additional chopped parsley if desired before serving.