Preheat the oven to 325 degrees F. Place a rack in the center of the oven.
Arrange the beets on a rimmed baking sheet and sprinkle with some kosher salt. Add 1 cup water to the pan, then cover the pan tightly with foil. Bake until the beets are completely tender, 60 to 90 minutes. Remove the beets from the pan and let cool.
Whisk together the lemon juice, mustard and vinegar in a medium bowl. Slowly whisk in 5 tablespoons of the olive oil.
Drizzle a platter with the remaining tablespoon of olive oil. Peel and thinly slice the beets. Arrange the beets on the platter, sprinkle with coarse salt and sugar to taste, and drizzle with the vinaigrette. Tear the smoked salmon into small pieces and scatter over the beets. Serve immediately.