Preheat the oven to 400 degrees F. Pierce the potatoes all over with a fork; microwave, turning once, until tender, about 15 minutes.
Melt 1 tablespoon butter in a large skillet over medium heat. Add the kielbasa and cook, turning, until lightly browned, 5 minutes. Transfer to a plate; cover to keep warm.
Melt 3 tablespoons butter in the skillet. Add the leeks and cook, stirring occasionally, until softened, about 2 minutes. Add the cabbage, 2 tablespoons water, 1/2 teaspoon salt and a few grinds of pepper. Cook, stirring occasionally, until the cabbage is tender, about 4 minutes.
Split the potatoes and arrange on a baking sheet. Top each with 1/2 tablespoon butter and 2 tablespoons Gruyere; fluff the insides with a fork to combine. Season with salt and pepper. Divide the kielbasa among the potatoes and top with the cabbage mixture. Toss the remaining 1/2 cup Gruyere with the parsley and sprinkle on top of the potatoes. Bake until the cheese is melted, about 10 minutes. Serve with mustard for dipping.
Photograph by Charles Masters
Courtesy of Food Network Magazine