Step 1

Combine salt, granulated sugar, brown sugar, peppercorns, coriander seeds, fennel seeds, and red pepper flakes in a medium bowl. Spread half of curing mix in the center of a foil-lined rimmed baking sheet roughly the same size as the fish. Place fish, skin side down, on curing mixture. Spread lemon zest, lime zest, and orange zest evenly over fish; cover with remaining curing mixture. Bring edges of foil up and over salmon and crimp to enclose. Place another baking sheet on top of salmon and weigh down with several large cans or a heavy pot. Chill, unwrapping and flipping fish halfway through, 24 hours.

Step 2

Rinse fish and pat dry; place, skin side down, on a cutting board. Using your longest, sharpest knife and wiping down blade with a moist towel between slices, cut on a diagonal ⅛"–¼" thick, leaving skin behind.

Step 3

Do Ahead: Salmon can be cured 3 days ahead. Cover tightly and chill.