Place chicken in a shallow dish or large zip-top plastic freezer bag.
Process quartered onion, 2 Tbsp. sugar, and next 7 ingredients in a blender or food processor until smooth, stopping to scrape down sides; pour over chicken. Cover or seal, and chill 2 hours.
Remove chicken from marinade, discarding marinade.
Sprinkle sesame seeds evenly over chicken; place in a lightly greased shallow roasting pan.