Preheat the oven to 450°F. Pierce the potatoes several times with a paring knife or fork and wrap each tightly in foil. Bake the potatoes until tender, about 1 hour. Set aside in the foil until cool enough to handle.

Meanwhile, slice the onions into ¼-inch rings and separate the pieces. Heat the oil in a large skillet over medium-high heat. Working in batches, add the onion rings to the skillet and sauté until golden, flipping once with tongs, about 2 minutes per side. Transfer the onion rings to a large bowl. Add the garlic to the skillet and cook until golden, about 1 minute. Transfer to the bowl with the onions. Let the oil cool slightly, then drizzle it over the onion mixture, season with salt and pepper, and set aside.

Meanwhile, remove the eggs from the refrigerator half an hour before cooking to bring them to room temperature. Bring a small pot of water that will just cover the eggs to a boil. Reduce the heat to a steady simmer and gently slide the eggs into the water. Simmer the eggs for 7 to 8 minutes until the yolks are just set. Remove the eggs from the heat and shock them in cold water to stop the cooking process.

To assemble the dish: Remove the potatoes from the foil and break them into large bite-sized pieces with your hands. Add the potatoes to the bowl with the sautéed onion mixture and toss to coat. Peel the eggs and cut them once lengthwise and once crosswise into quarters. Add them to the bowl with the onions and potatoes. Fold in the scallions and season the mixture with salt and pepper to taste. Serve at room temperature with smoked fish.