Heat the olive oil in a sauté pan over medium-high heat. Add the chicken and cook for about 5 minutes per side, until cooked through, not moving the chicken too much.
While the chicken is cooking, in a small bowl, combine the sake, water, soy sauce, rice vinegar, sugar, ginger, sesame oil, and garlic.
Remove the chicken from the pan.
Add the sauce to the pan, turn down the heat to low, and simmer for 2 minutes.
Return the chicken to the pan with the scallions and simmer, turning the chicken once to coat in the sauce, for an additional 5 minutes.