Preheat oven to 350°. In a large skillet, heat butter over medium heat. Add sausage and onion; cook and stir until onion is tender, 8-10 minutes. Stir in sauerkraut; cool slightly.
Prepare the Rosette's Dough Mix according the the directions under "Bake This Way", one package will make two strombolis. After kneading the dough separate into two balls and roll each ball out into a 1/4" thick rectangle on parchment paper.
In a small bowl, mix sour cream, mustard and 1/2 teaspoon caraway seeds. . Spread with 1/2 cup sour cream mixture to within 1/2 in. of edges. Spoon half of the sausage mix into the center of rectangle dough; sprinkle with 1/2 cup each mozzarella and cheddar cheeses.
Grab the edge of the parchment paper (on the long side of the dough) and use it to roll the dough over the ingredients and press the edges together Repeat with with the other stromboli.
Sprinkle with remaining caraway seeds. Cut slits in pastry. Bake until golden brown, 25-30 minutes. Let stand 10 minutes before slicing.