How to Make It
In a large saucepan, cover the potatoes with water and bring to a boil. Cook over moderately high heat until just tender when pierced with a knife, about 15 minutes. Drain and let cool. Cut each potato into 4 slices.
Meanwhile, in a large skillet, melt the butter. Add the red onion and cook over moderate heat, stirring occasionally, until softened, about 7 minutes. Add the cabbage and cook over low heat, stirring occasionally, until tender and starting to brown, about 15 minutes. Add the paprika and cook over moderate heat, stirring, until fragrant, about 2 minutes. Add the tomatoes and cook until heated through, about 2 minutes. Season with salt and pepper.
Light a grill. In a bowl, mix the sour cream with the scallions and season with salt and pepper.
Prepare four 12-by-17-inch sheets of heavy-duty foil. On a work surface, fold up the sides up of each sheet to form a 7-inch square box, with 2 sides higher than the others. Arrange the kielbasa slices in a circle in the center of each box. Arrange the potato slices over and around the kielbasa. Season the potatoes with salt. Mound the cabbage mixture over the kielbasa, then pour cup of the water into each package. Bring the 2 higher sides of each package together and fold them over, then fold in the sides to seal.
Grill the packets over high heat until bubbling inside, about 5 minutes. Carefully open the packages and serve, passing the scallion sour cream at the table.