Season the Salmon Fillets with Traeger Salmon Shake and let rest, uncovered, in the refrigerator for 3 hours.
When ready to cook, start the Traeger grill on Smoke with the lid open until the fire is established (4 to 5 minutes).
Leave on the Smoke setting. Arrange the Salmon (skin-side down, if it has skin) on the grill grate.
Smoke for 2 to 2-1/2 hours or until the fish flakes easily with a fork. (The exact time will depend on the thickness of the fillet.) There is no need to turn the fish.
Using a large thin spatula, transfer the Salmon to a wire rack to cool. Cover the fish with plastic wrap and refrigerate if not using immediately. (Can be made 2 to 3 days ahead.)
Break into flakes, discarding any skin or discolored parts.
Increase the temperature on the Traeger to 400F.
Oil a rimmed baking pan with the olive oil and arrange the potato slices on the pan in a single layer.
Roast the potatoes for 15 to 20 minutes, or until tender and lightly browned, Let cool.
Transfer to a large mixing bowl and add the Salmon, bread cubes, and onions, and stir gently with a rubber spatula.
Generously butter a 9- by 13-inch baking pan, preferably nonstick.
Transfer the bread and potato mixture to the baking pan. In another large mixing bowl, whisk the eggs, milk, dill, mustard, and salt and black pepper. Pour evenly over the bread and potato mixture.
Let sit for 30 minutes before baking, or up to 1 day ahead, covered and refrigerated. (Uncover and bring to room temperature before baking.)
When ready to bake the casserole, start the Traeger grill on Smoke with the lid open until the fire is established (4 to 5 minutes).
Set the temperature to 350F and preheat, lid closed, for 10 to 15 minutes.
Bake the casserole until the egg mixture is set and the top is very lightly browned, 45 minutes to an hour. (A knife inserted in the center should come out clean.)
Allow to cool for 15 minutes before serving. Cut into squares with a sharp knife. Serve sour cream, diced red onions, lemon wedges, and capers in separate bowls on the side.