Whisk together fish sauce, lime juice, lemongrass, agave, soy sauce, ginger, chilli flakes and garlic. Pour 3/4 of the marinade into a shallow baking dish; reserve the remaining marinade. Thinly slice chicken crosswise, with your knife slightly on the diagonal. Add the chicken to the marinade in the baking dish, cover with cling film and refrigerate for 2 hours.
Strain reserved marinade into a large bowl; whisk with the vegetable oil. Add the shredded cabbage, snap peas and coriander; cover with plastic and refrigerate until ready to serve.
Preheat a grill or grill pan to medium-high; soak about 24 skewers in water for 20 minutes. Thread chicken onto skewers. Lightly oil grill grates; sear or grill chicken in batches until cooked through, about 3 to 4 minutes per side. Toss slaw; serve with chicken and Spicy Peanut Sauce, if desired.