Heat 2 Tbsp. oil in a large skillet over medium. Add onions, season with salt, and cover. Cook, uncovering and stirring occasionally, until onions are softened and just beginning to take on color, about 5 minutes. Uncover, add ginger, and cook, stirring occasionally, until onions are jammy and golden brown, 10–12 minutes. Stir in soy sauce. Transfer onion mixture to a plate and wipe out skillet. Pour remaining 1 Tbsp. oil into same skillet and increase heat to medium-high. Add potatoes in a single layer and season with salt. Cook, turning halfway through, until golden brown, 10–12 minutes. Spread onion mixture over potatoes, leaving a 1" border of potatoes without onion mixture. Season fish with salt and place on top of onion mixture. Cover and reduce heat to medium-low. Cook until fish is opaque throughout and easily flakes apart, 10–12 minutes. Serve topped with scallions and furikake (if using).