Start by washing the russet potatoes thoroughly. Halve them, and if they are large, quarter them for quicker cooking. Next, quarter the Napa cabbage and remove the thick core. Trim and quarter the fennel bulb, and slice the red onion into quarters as well.
Place the halved russet potatoes in a pot of salted water and bring to a boil. Cook until fork-tender, about 15-20 minutes. Drain and allow them to cool slightly before dicing them into bite-sized pieces.
While the potatoes are cooking, thinly slice the Napa cabbage, fennel, and red onion. Aim for uniform pieces to ensure even mixing and flavor distribution.
In a small bowl, whisk together mayonnaise, sour cream, reserved caper juice, Dijon mustard, sugar, ground cayenne pepper, and salt and pepper to taste. Adjust the seasoning according to your preference.
In a large mixing bowl, combine the diced potatoes, sliced Napa cabbage, fennel, red onion, chopped parsley, and capers. Pour the dressing over the mixture and toss until everything is well coated.
For the best flavor, let your Seriously Awesome Slaws chill in the refrigerator for at least 30 minutes before serving. This allows the flavors to meld beautifully!