Arctic Char Gravlaks with Cucumber Jelly

Name: Arctic Char Gravlaks with Cucumber Jelly
Ingredients: 1 (1 1/4-lb) center-cut piece arctic char fillet with skin, pin bones removed; 1/2 cup sugar; 1/4 cup kosher salt; 1 tablespoon coarsely ground black pepper; 3 cups coarsely chopped fresh dill (from 2 large bunches); 3 seedless cucumbers (usually plastic-wrapped; 3 lb total); 3/4 teaspoon table salt; 2 teaspoons unflavored gelatin (from 1 envelope); 1 tablespoon distilled white vinegar; 1 tablespoon tiny fresh dill fronds; Accompaniment: thin Scandinavian crispbread such as Kavli