Sprinkle the salmon fillet with salt and let stand for 2 hours.
Meanwhile, wash the rice thoroughly in cold water 30 to 60 minutes before cooking and let drain in colander. Place rice and water in a heavy, tightly covered pot over mediumhigh heat. When water just begins to boil, turn the heat to high and let it come to a vigorous boil. Reduce the heat to low and cook until all the liquid is absorbed by the rice, about 12 to 13 minutes. Turn off the heat and let the rice stand, covered, for 10 to 15 minutes. Using a flat wooden spoon or rice paddle, fluff the rice with a cutting motion. Stretch a towel under the lid and cover tightly to keep warm until ready to use.
Toast the nori sheets over a high gas flame, and cut crosswise into 1inch wide strips, or use pretoasted nori.
Mix the bonito flakes with the soy sauce. Rinse the salt off the salmon, pat dry, and grill for 3 to 5 minutes. Use a fork to break the salmon into small pieces.
Wet your hands with salted water to keep the rice from sticking to your hands. Cup one hand and place a handful of rice, about 1/2 cup, in your hand. Make an indentation in the rice and tuck in one of the fillings: a teaspoon of soaked bonito flakes, a few flakes of salmon, or a few pieces of pickled plum. Close the rice over the filling and mold it into a triangular shape. Mold the rice firmly, pressing just hard enough to hold it together. Set the rice triangle down on one of its sides and cover the top peak with a strip of nori, shiny side out, like a roof. You can also make cylindrical shapes and wrap the nori around the middle. Sprinkle sesame seeds over the rice shapes. These are great lunch treats.