1. Drain salmon. Transfer to a bowl and remove and discard any skin and bones; flake with a fork.
2. In another bowl, whisk eggs, milk, mustard, sugar, and salt until blended.
3. Peel potatoes and cut crosswise into very thin (1/16-in.-thick) slices. Arrange about a third of the potato slices over the bottom of a 9- by 13-inch baking dish. Sprinkle half the salmon evenly over potatoes, followed by half the dill. Repeat layers, ending with potatoes. Pour milk mixture over potatoes and distribute butter chunks evenly over the top. Sprinkle generously with pepper and cover dish tightly with foil.
4. Bake in a 375° regular or convection oven until potatoes are tender when pierced, about 45 minutes. Remove foil and continue to bake, uncovered, until golden brown on top, 15 to 20 minutes.