1/2 lb sliced pumpernickel bread, cut into triangles
Directions
Peel potatoes and cut into 1/4-inch-thick slices, then transfer to a 2-quart saucepan and cover with water by 1 inch. Add 1/2 teaspoon salt and bring to a boil. Reduce heat and simmer, partially covered, until potatoes are tender, 5 to 8 minutes. Drain, then transfer to a bowl.
Meanwhile, whisk together lemon juice, chopped capers, mustard, sugar, and remaining 3/4 teaspoon salt until sugar and salt are dissolved, then whisk in oil until emulsified.
Toss potatoes gently with 1/4 cup dressing. Toss onion with 1/4 cup dressing in another bowl and let stand 5 minutes.
Peel and pit avocado, then cut into 1/4-inch-thick slices.
Arrange potatoes, avocado, cucumber, onion, salmon, and bread on a large platter and sprinkle with whole capers. Drizzle with some dressing and serve remainder on the side.
