Toss chicken, garlic, ginger and 1 tablespoon fish sauce in a bowl until the chicken is coated in the marinade. Cover and marinate for 1 hour.
While the chicken is marinating, heat the peanut oil in a frypan over medium heat. Stir in onions, garlic, ginger and chillies. Cook and stir until the onion begins to turn golden brown, about 7 minutes. Reduce heat to low and stir in peanut butter, coconut milk, soy sauce, brown sugar, 1 1/2 tablespoons fish sauce and lime juice until blended. Simmer very gently for 10 minutes, then remove from heat and keep warm.
Preheat a barbecue for medium-high heat and lightly oil grate. Thread chicken onto skewers.
Barbecue chicken skewers on preheated barbecue until no longer pink in the centre, about 3 minutes per side. Serve with warm peanut sauce.