This Scandinavian-influenced gratin is a luxurious meld of smoked salmon, red potatoes, and leeks. Fresh horseradish enlivens a toasted dill-crumb topping, but be aware when grating it that tears may flow.
Preheat oven to 375 degrees. Butter a 9-inch square baking dish. Stir together bread pieces, 1 tablespoon dill, the horseradish, 1/4 teaspoon salt, and a pinch of pepper in a medium bowl. Melt 2 tablespoons butter; stir into bread mixture. Set aside.
Melt remaining 2 tablespoons butter in a large skillet over medium heat. Add leeks; cook, stirring occasionally, until very tender, about 5 minutes. Transfer to a bowl, and stir in remaining 3 tablespoons dill; set aside.
Toss potatoes with 1/2 teaspoon salt and 1/4 teaspoon pepper in a large bowl. Arrange one-third of potatoes in buttered dish, overlapping slices. Top with half of leek mixture, and then half of the smoked salmon. Add one-third of the potatoes, and one more layer each of remaining leek mixture and smoked salmon. Top with remaining potatoes. Pour stock over layers; gently shake dish back and forth to distribute evenly. Sprinkle with bread mixture.
Cover with foil; bake 30 minutes. Remove foil; bake until potatoes are very tender and top is golden, 30 to 40 minutes more. Serve warm, garnished with a dollop of creme fraiche, if desired.