Discard the top two thirds of the lemongrass as well as one inch at the bottom; peel off the tough outer layer and then smash with a meat mallet and finely chop. Add to a large bowl along with the garlic, fish sauce, soy sauce, sugar, avocado oil, and pepper. Mix, then add the chicken thighs and toss to coat. Marinate for at least 30 mins (or up to 12 hours in the fridge).

In a large, deep skillet (or pot) over medium-high heat, heat the avocado oil until shimmering. Add the chicken in a single layer and sear until golden brown, about 3-4 mins per side. Remove from the heat and transfer the chicken to a plate.

Return the pan to the stove over medium heat. Add the shallot and chili and cook, stirring, until softened, 2-3 mins. Stir in coconut milk and stock, scraping up any browned bits. Add the fish sauce, salt and pepper and bring to a simmer over med-high heat.

Return chicken to the pan along with any juices. Nestle in bok choy around the chicken, lower heat to a gentle simmer, cover and cook 6-9 mins until the bok choy is just wilted and bright green and the chicken is cooked through. Stir in the juice of half a lime, taste it, and add the other half if you'd like the broth tangier.

Serve the chicken and bok choy in shallow bowls alongside white rice. Spoon the broth over the top, garnish with cilantro and sliced chili and enjoy!