To prepare rice, rinse rice thoroughly in a sieve.
Drain well. Bring 2 cups water to a boil in a medium saucepan; add rice and salt. Cover, reduce heat, and simmer 20 minutes or until liquid is absorbed.
Remove from heat; uncover and cool to room temperature.
To prepare dressing, combine vinegar and next 5 ingredients (vinegar through garlic) in a small bowl.
Add wasabi, if desired.
Combine cooled rice, dressing, cucumber, onion, and sesame seeds in a large bowl.