LINE a large baking sheet with 2 sheets of plastic wrap, with enough overhanging to cover the whole sheet. Stir sugar with salt in a bowl. Sprinkle half of sugar mixture on prepared sheet.
REMOVE salmon from freezer. Trim and discard skin if attached. Slice salmon about 1/8 in. thick. Arrange salmon slices in 1 layer over sugar mixture. Completely cover salmon with remaining sugar mixture. Wrap fish with overhanging plastic wrap and refrigerate 2 hours.
BOIL rice with water in a medium saucepan, then reduce heat to low. Simmer, covered, until liquid is absorbed, 15 to 20 min. Drizzle with vinegar and sesame oil. Stir until well coated, then spread evenly on a plate. Chill in freezer for 5 min. Sprinkle with panko and stir until combined. Using wet hands, scoop heaping 1-tbsp portions of rice and form into balls. Arrange on a platter.
TRANSFER salmon to a colander. Rinse slices under cold running water until sugar mixture is completely removed. Pat salmon dry with paper towel. Top onigiri rice balls with salmon. Sprinkle with spice blend. Garnish with dill.