Unsalted butter adds a creamy richness to the lean whitefish and potato filling, making it easier to spread into the cabbage leaves, and keeping the rolls moist through browning and simmering.
Bring potatoes to a boil in a large saucepan of lightly salted water over high. Cook until easily pierced with a paring knife, about 25 minutes. Drain and let cool 25 minutes. Peel potatoes, and cut into 1/2-inch pieces; place in a large bowl. Add whitefish, dill, room-temperature butter, egg, chives, salt, and white pepper; gently stir to combine. Set filling aside.
Bring a medium stockpot of salted water to a boil over high. Carefully remove about 20 large outer cabbage leaves in bunches (so they are not completely separated), as well as 3 to 5 smaller leaves (to patch up tears in the larger leaves). Working in 2 batches, add cabbage leaves to boiling water; cook until leaves are pliable and can bend easily without breaking, about 10 minutes per batch. Using a spider, transfer leaves to a wire rack set inside a rimmed baking sheet to drain. Let cool 15 minutes. Separate large cabbage leaves; use a paring knife to carefully shave veins to make it easier to roll the filled cabbage leaves.
Lay cabbage leaves on a work surface, cupped side up. Spoon 1/4 cup packed potato filling onto 1 end of each cabbage leaf. Starting at end with filling, roll up leaf slightly, and fold outer sides of leaf over toward center, keeping leaf pressed tightly around filling as you roll into a neat cylinder. Place cabbage rolls, seam sides down, on a plate.
Heat 1 tablespoon oil in a large high-sided skillet over medium. Add half of cabbage rolls, seam sides down. Cook, undisturbed, until browned, 4 to 5 minutes. Carefully flip rolls, keeping seams intact. Add 1 cup fish stock and 1 tablespoon cold butter, gently swirling skillet to distribute. Return to a simmer. Reduce heat to low, and gently simmer, undisturbed, until thickest part of vein is easily pierced with a paring knife, 5 to 9 minutes. Transfer rolls to a platter; cover to keep warm. Wipe skillet clean. Repeat process with remaining 1 tablespoon oil, remaining cabbage rolls, remaining 1 cup fish stock, and remaining 1 tablespoon cold butter. Sprinkle cabbage rolls with dill leaves. Serve with sour cream and pickled red onions.
Filling can be prepared and stored in an airtight container in refrigerator up to 3 days. Let come to room temperature for 1 hour before using to avoid tearing cabbage leaves.
If smoked whitefish isn’t available, substitute cold-smoked salmon.