Our modern take on the Passover classic transforms this often divisive dish into elegant quenelles made with salmon and cod rather than the traditional carp or pike.
Heat oil in a large straight-sided skillet over medium. Add chopped onion and carrots; season with salt and cook, stirring occasionally, until very soft, about 10 minutes. Transfer mixture to a bowl; let cool.
Return skillet to medium-high heat (do not wipe clean). Add remaining onion and carrot, celery, bay leaf, and peppercorns; toast until fragrant, about 2 minutes. Add 8 cups water, fish sauce, and 1 tablespoon salt; bring to a boil. Remove from heat. In a small bowl, combine 1/3 cup liquid and matzo meal. Cover skillet.
Meanwhile, slice 1/2 pound salmon into 10 strips, each about 1 inch thick and 2 inches long. Place on a plate, season with salt, cover, and refrigerate. Chop remaining 1 pound salmon into 1-inch pieces. Place chopped salmon and cod in a food processor with chopped-onion mixture, matzo-meal mixture, mustard, sugar, and dill. Add 2 1/2 teaspoons salt and 1/2 teaspoon pepper; pulse until combined. In the bowl of a mixer fitted with the paddle attachment, beat eggs on medium speed until foamy, 1 minute. Add fish mixture; beat until fluffy, about 7 minutes. Cover and refrigerate 1 to 4 hours.
Scoop fish mixture into 10 half-cup balls; transfer to a parchment-lined rimmed baking sheet. With dampened hands, press a salmon strip into each ball, then shape into an oval. Return liquid in skillet to a simmer. With a spoon, gently drop in fish ovals. Remove from heat; cover and let stand 15 minutes. Refrigerate overnight.
Let stand at room temperature about 30 minutes. With a slotted spoon, gently remove gefilte fish. Serve over escarole with lemons, cucumbers, radishes, matzos, and sauces.
In some Jewish traditions mustard is considered kitniyot and not allowed for Passover.