Step 1 Toss the chicken with the sake, onion and ginger in a shallow, heat-proof bowl large enough to hold the chicken.
Step 2 Prepare a stove-top steamer and place the chicken, still in the bowl, on the rack in the steamer. Steam the chicken until the meat is cooked through; it will be firm and opaque and the juices will run clear, 20 to 25 minutes.
Step 3Remove from heat and cool the chicken in the bowl with the flavorings, then refrigerate the chicken, still in the bowl, until completely chilled. The chicken can be cooked up to 1 day ahead and refrigerated before shredding.
Step 4Shred the chicken into matchstick pieces about 3 inches long. Place the chicken in a medium bowl and toss with the sesame oil. Refrigerate the chicken, covered, until ready to use.