Add the sweet potatoes to a large pot and add enough water to cover the sweet potatoes. Add a pinch of salt and bring to a boil. The potatoes are done when easily pierced with a knife, about 15-20 minutes.
While the potatoes are cooking, rinse the chicken thighs and pat dry. Then season on both sides with turmeric, sea salt, and pepper.
Drain potatoes and return to the pot. Add 4 tbsp. of butter and mash with a potato masher. Set aside.
Add 1 Tbsp. of the butter to a medium pot and heat over medium high heat. Once the butter is hot, add the onions. Stir onions and cook until translucent. Remove the onions from pan and set aside.
Add the remaining 1 Tbsp. of the butter back to the medium pot on medium-high heat and once the butter is hot, add the chicken. Brown the chicken on both sides. Continue cooking and add the coconut milk, coconut aminos, and coconut vinegar and mix. Then add the onions, the chili flakes, and 5 Tbsp. of the lemongrass paste. Continue cooking for about 10 minutes on medium heat.
To serve, place a large scoop of sweet potato mash in a bowl. Then top with chicken and lemongrass gravy mixture and garnish with cilantro and black sesame seeds. Enjoy!