Mix together first 4 ingredients in a small bowl, and set aside.
Heat a large skillet (not nonstick) over high heat, and swirl in 1 tablespoon oil. Toss in half the chicken and a pinch of salt and pepper, and brown on all sides, about 4-5 minutes total. Transfer to a large bowl. Swirl in a half-tablespoon oil, and brown the remaining chicken with another pinch of salt and pepper, then add to the chicken in the bowl.
Swirl in the remaining oil, and saute the onion and garlic for 1 minute, until fragrant. Add the sugar snap peas to the pan, and toss well. Stir-fry for 5-7 minutes, until peas are seared in places, but not yet tender. Pour in mirin, and scrape up bits stuck to the bottom of the pan. Cook for 2 minutes, until liquid has almost completely evaporated.
Add chicken back to the pan, and toss with peas. Stir in sauce, and cook for 2 minutes, stirring and flipping frequently, until sauce has thickened slightly and lightly coats the chicken and vegetables.
Serve sprinkled with cilantro.