Potato and Smoked Fish Cabbage Rolls

Name: Potato and Smoked Fish Cabbage Rolls
Ingredients: 1 3/4 pounds russet potatoes; 10 ounces smoked boneless, skinless whitefish, broken into bite-size pieces (about 2 cups); 1/2 cup chopped fresh dill, plus leaves for garnish; 1/4 cup unsalted butter, at room temperature, plus 2 tablespoons cold unsalted butter, divided; 1 large egg, beaten; 1 tablespoon thinly sliced fresh chives; 1 1/2 teaspoons kosher salt, plus more; 1/8 teaspoon ground white pepper; 2 large heads green cabbage, halved lengthwise and cored; 2 tablespoons canola oil, divided; 2 cups fish stock (such as Bar Harbor), divided; Sour cream and pickled red onions, for serving