Pour onion juice into a 4 cup glass measure. Add salt and sugar; stir to dissolve. Stir in pepper and the coarsely chopped dill.
Place salmon in 2-gallon zipper-lock bag. Pour in marinade, seal bag, and place on a jelly roll pan.Marinate should barely cover fish. Place second jelly roll pan over fish and set about 7 pounds of weight (heavy cans or bricks work well) on top of pan. Refrigerate weighted fish until very firm, 12 to 18 hours, depending on the thickness of the fish.
Remove salmon fillet from bag and dry with paper towels. Evenly distribute the minced dill over belly side of the fillet, then press dill into flesh. (Gravlax can be wrapped in parchment paper, then plastic wrap and refrigerated up to 1 week.)
Just before serving, place fillet, belly side up, on work surface. Slice desired amount of gravlax. Rewrap and refrigerate unsliced gravlax for later use.