Make the salmon: Pulse the coriander and fennel seeds in a spice grinder until coarsely ground. Transfer all but 1 teaspoon of the spice mixture to a small bowl and stir in the sugar and 2 teaspoons salt (reserve the remaining 1 teaspoon spice mixture for the sauce).
Line a rimmed baking sheet with foil. Place the salmon skin-side down on the foil. Rub the salmon with the olive oil, then sprinkle with the sugar-spice mixture. Refrigerate, uncovered, 1 hour.
Meanwhile, make the herb sauce: Puree the parsley, chervil, chives, lemon zest and juice and the reserved 1 teaspoon spice mixture in a blender until a coarse paste forms. With the machine running, gradually add the olive oil and 1/4 cup cold water and puree until smooth. Season with salt and pepper.
Preheat the broiler. Pat the salmon dry with paper towels. Broil the salmon until the top is browned and a thermometer inserted into the center registers 110˚, 9 to 11 minutes. Let rest 10 minutes on the pan. Transfer the salmon to a serving platter with 2 large spatulas, removing the skin. Serve with the herb sauce.