How to Make It
Stir together sugar, 1/2 cup kosher salt, coriander, and black pepper in a small bowl. Spread about 1/4 cup sugar mixture on a plate in an even layer about the size of 1 trout fillet. Place 1 fillet, skin side down, on sugar mixture on plate; top with 1/2 cup chopped cilantro and 1/4 cup sugar mixture. Top with remaining fillet, flesh side down; sprinkle with remaining 1/2 cup sugar mixture and remaining 1/4 cup chopped cilantro. Wrap plate tightly with plastic wrap. Chill until fish is slightly firm, about 8 hours.
Unwrap fillets, and brush off sugar mixture and cilantro. Rinse fillets under cold water; pat dry with paper towels. Process 1/2 cup cucumber, about three-fourths of jalapeño, lime juice, and 1 tablespoon water in a blender until smooth, about 20 seconds. Pour mixture through a fine wire-mesh strainer into a bowl; discard solids. Stir in remaining 1/2 teaspoon kosher salt. Chill until ready to serve.
Pour chilled cucumber-lime water on a rimmed serving platter. Thinly slice trout crosswise; discard skin. Arrange trout pieces on platter. Top with spring onion, avocado, remaining 1 1/2 cups cucumber, and remaining jalapeño. Sprinkle with flaky sea salt and cilantro leaves. Serve immediately with lime wedges and tostadas.