For the chicken skewers, first cut chicken fillet into strips about 2 cm wide. Thread onto oiled wooden skewers in the shape of a concertina. Place in a deep plate.
Place garlic, onion and half each of the sugar and fish sauce in a hand blender, blend to a smooth paste and thin with cold water. Pour over meat, cover with foil and let marinate for 3-4 hours, turning occasionally to coat the other side.
Cook remaining ingredients in a small saucepan over medium heat until thick sauce forms, about 4 minutes.
Brush meat skewers with oil and cook on charcoal grill or in oven over high heat until golden brown. Turn the chicken skewers frequently to the other side and brush with oil often if necessary.