Place the chicken thighs into a bowl with the sea salt, onion powder and Vietnamese curry powder. Mix well and leave to marinate for at least 20 minutes.
Heat a pan over a medium heat and add the vegetable oil.
Add the onion and cook for 2–3 minutes until softened. Stir in the garlic and cook for 1 minute more.
Add the lemongrass and bay leaf, if using, followed by the marinated chicken. Cook until lightly coloured.
Add the potato and carrot, stirring well to coat in the spices.
Pour in the coconut milk and add a little water if needed to loosen the sauce.
Add the sugar, if using, and bring to a gentle simmer.
Cook for 20–25 minutes until the vegetables are tender and the chicken is cooked through.
Remove the lemongrass and bay leaf before serving.