Arctic Char Gravlaks with Cucumber Jelly

Pat fish dry, then transfer, skin side up, to a large sheet of plastic wrap. Stir together sugar, salt, and pepper, then rub 3 tablespoons of mixture onto skin of fish. Turn fish over and thickly coat with remaining sugar mixture, then pack dill on top.
Wrap fish tightly in 2 or 3 layers of plastic wrap (to prevent leakage; salt mixture will liquefy as fish cures) and transfer to a large shallow baking pan. Put another baking pan or a cutting board on top of fish and weight down with 3 or 4 full cans (about 3 lb total). Let fish cure, chilled, turning wrapped fillet over roughly every 12 hours and then replacing weight, for 36 hours total.
Peel cucumbers, making sure to remove all green (for a clearer jelly), then halve lengthwise and core. Coarsely chop cucumbers and purée in a food processor until smooth, then drain in a large fine-mesh sieve set over a bowl, pressing hard on solids to extract 2 cups liquid. Discard solids.
Stir together salt and 1/2 cup cucumber liquid in a small saucepan and sprinkle with gelatin. Let stand 1 minute to soften, then heat over moderate heat, stirring, just until gelatin is dissolved, about 2 minutes. Cool mixture to room temperature, then stir into remaining 1 1/2 cups cucumber liquid along with vinegar. Pour mixture into an 8-inch square glass baking dish and sprinkle with dill fronds, pressing gently to submerge. Chill, covered, until set, at least 8 hours.
Unwrap gravlaks, discarding liquid, and gently scrape off dill. Transfer gravlaks, skin side down, to a cutting board. Holding a very sharp long thin-bladed knife at a 30-degree angle, cut gravlaks across the grain into very thin slices, being careful not to cut through skin. Discard skin.
Cut jelly into 6 pieces and divide among 6 plates with a metal spatula. Serve with several slices of gravlaks.