In a large glass 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 a barbecue 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. Barbecue 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.
Slice beef into thin slices and tenderize with a fork. Trim away any fat and gristle. Marinate, slice and thread onto skewers as for chicken.
Make extra marinade, reduce in a saucepan over high heat for about 5 minutes, and strain to make a dipping sauce for the kebabs.