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.