How to Make It
Finely grate the lemon zest and reserve it. Using a knife, peel the lemon, removing all of the bitter white pith. Working over a small saucepan, cut in between the membranes to release the sections. Squeeze any remaining juice from the membranes into the saucepan and bring to a gentle simmer. Cook over moderately low heat, stirring occasionally, until the lemon breaks down, about 15 minutes. Transfer to a small bowl and stir in the olive oil and preserved lemon. Season the vinaigrette with salt; let cool.
Meanwhile, in a large skillet, heat 1 tablespoon of the canola oil. Add the panko and fennel seeds and toast over moderately low heat, stirring occasionally, until golden and fragrant, about 5 minutes. Stir in the dill and season with salt. Transfer the breadcrumbs to a paper towel–lined plate to drain. Wipe out the skillet.
In a small bowl, combine the grapefruit and lime zests with the reserved lemon zest, the sugar and 3 tablespoons of kosher salt. Lay the trout fillets on a baking sheet ski side down and cover evenly with the citrus-salt cure. Let stand for 6 minutes. Rinse the trout and pat dry.
In the large skillet, heat 2 tablespoons of the canola oil. Lightly season the trout with salt and pepper. Add 2 fillets to the skillet skin side down and cook over moderately high heat until the skin is golden and crispy, about 3 minutes. Flip the fish and cook until golden and white throughout, 2 to 3 minutes longer. Briefly drain the fish on paper towels, then transfer to plates. Repeat with the remaining 2 tablespoons of oil and 2 trout fillets. Spoon some of the lemon vinaigrette over the trout and garnish with the fennel seed breadcrumbs. Serve with braised endives and pass the remaining vinaigrette at the table.
Note: Jarred preserved lemon can be found at specialty food shops and stores like Whole Foods.