Remove the woody root and outer layers of the lemongrass and chop the stalks into small pieces.
Peeled and roughly chop the red onion and garlic; slice the fresh ginger into small pieces.
Add the chopped lemongrass, red onion, garlic and ginger to a food processor and pulse until you have a thick pulp.
Add ground coriander, ground fennel, turmeric, brown sugar, salt and coconut oil to the pulp.
Blitz again until the mixture becomes a smooth, thick paste; add up to 1 tablespoon water if the processor struggles to blend.
Cut the chicken thighs into large chunks and place them in a bowl or lidded container.
Pour the marinade over the chicken and toss to coat thoroughly, then thread the marinated chicken onto wooden skewers.
Cover and refrigerate the skewers overnight to marinate; alternatively freeze them and use when needed.
Preheat a grill or grill pan to high heat. Cook the skewers 2–3 minutes per side until cooked through and charred in places.
Remove from the heat and let the skewers rest for a few minutes before serving.