Rinse the fillets in cold water and pat them dry with paper towels. Combine the salt and sugar, and rub the flesh side of the fish with the mixture. Place one fillet skin side down in a deep dish just big enough to hold the fillets. Scatter the pepper, fresh dill, and dill seeds over it. Place the other fillet skin side up on top. Cover the dish with plastic wrap and place a heavy weight, such as two heavy dishes or a saucepan, on top of the fish. Refrigerate for 3 to 4 days, turning the fish every 12 hours and basting it with the brine that accumulates in the dish. To serve, discard the brining liquid and brush off the dill. Slice the fish into thin slices on the diagonal with a sharp thin knife. The flesh from the tail will be leaner than the flesh from the belly. Serve with the mustard sauce.