In a large nonreactive bowl, stir together the onion, garlic, lemon grass, soy sauce, peanut oil, turmeric, brown sugar, cumin, ginger, salt, and pepper. Reserve about 2 tablespoons of marinade in a small bowl, and refrigerate until cooking time. Mix the chicken breasts into the remaining marinade, stir to coat well, and marinate in refrigerator overnight.
About 30 minutes before serving, soak the skewers in water. Preheat an outdoor grill for high heat, and lightly oil the grate.
Remove the chicken breasts from the bowl, and discard the used marinade. Cut each chicken breast on the diagonal into 6 long strips, and thread a chicken strip onto one of the soaked skewers. Grill the skewers, turning frequently and basting with the reserved marinade, until the chicken is cooked through with brown, crispy edges, 5 to 8 minutes per skewer.