Heat rice according to package directions; spread on a plate.
Sprinkle with 1 1/2 teaspoons vinegar, sugar, and salt; cool
completely.
Combine remaining 1/2 teaspoon vinegar, onions, soy sauce, oil,
and chicken in a small bowl.
Arrange 4 (6-inch) squares of plastic wrap on a work surface. Divide
rice mixture into 4 equal portions, shaping each into a ball. Lightly
press each rice ball into a disc between palms; place 1 disc on each
plastic wrap square. Make an indentation in each. Top each
indentation with one-fourth of chicken mixture. Working with one
rice ball at a time, lightly press the rice over the filling. Gather up
ends of plastic wrap, and twist tightly to form a ball. Gently remove
plastic wrap; decorate with nori, if desired.
