Thai BBQ Chicken (Gai Yang)

Name: Thai BBQ Chicken (Gai Yang)
Prep time: 2 h 15 min
Cook time: 15 min
Total time: 2 h 30 min
Cuisine: Thai
Ingredients: 1 shallot; 1 lemongrass; 4 cloves garlic; small handful coriander roots; 1 tsp palm sugar; Juice of 1 lime; 1 tbsp neutral flavoured oil (like peanut); 1 tbsp Thai dark soy sauce; 1 tsp rice vinegar; 1 tbsp fish sauce; ground white pepper, to season; 8 chicken thighs, boneless, skinless; steamed rice, sliced cucumber, to serve; 3 to 4 red Thai chilies; 120g caster sugar; 120ml rice vinegar; 120ml water, + 20ml extra for slurry; 1 tsp tapioca starch; salt, to season