Put the potatoes in a large pot and add water to cover and 1 teaspoon of the salt. Bring to a boil, reduce the heat to medium-high, and cook until fork-tender, about 12 minutes. Remove from the heat, drain, and let cool to room temperature.
Trim about 1/4 inch off each end of the potatoes. Using a paring knife or a small spoon, scoop out about three-quarters of the flesh of each potato from one end and reserve in a bowl. Place the potato shells on a parchment paper–lined baking sheet or work surface.
Season the salmon on the flesh side with 1/4 teaspoon of the salt, 1/8 teaspoon of the black pepper, and the sugar. Heat a medium-size nonstick skillet over medium- high heat. Place the salmon flesh side down in the hot skillet and cook for 3 minutes, then flip it over and cook on the other side for 2 minutes. Remove from the heat and let cool to room temperature, then flake the salmon with a fork.
With a fork, mash the reserved potato flesh. Add the onion, chervil, olive oil, 1/2 teaspoon of the salt, and the remaining 1/2 teaspoon black pepper. Add the flaked fish and stir to mix.
Sprinkle the potato shells with the remaining 1/4 teaspoon salt and 1/8 teaspoon pepper. Spoon the mixture into the shells and serve at room temperature.
Yield: 12 servings (2 potatoes per person)