In a large bowl or airtight container, whisk together warm water, fish sauce, and sugar until the sugar is fully dissolved.

Peel the tough outer layers off the lemongrass stalks to reveal the tender core, trim both ends, then bruise the stalks with a heavy object to help release their oils. Finely mince or pulse in a food processor to get about half a cup.

Combine the chopped lemongrass, scallions, garlic, and white pepper into the fish sauce mixture, stirring well.

Pat dry the chicken thighs with paper towels, remove excess fat, and cut each thigh into halves. Add to the marinade and toss thoroughly to coat all pieces, ensuring the chicken is submerged. Cover and chill in the fridge for at least 12 hours and up to 24 hours.

For stovetop cooking: Remove chicken from marinade and scrape off excess lemongrass bits as much as possible but some residue can remain.

Heat half of the neutral oil in a large nonstick skillet over medium-high heat until shimmering but not smoking. Add half the chicken pieces in one layer and cook until the exterior turns golden and the internal temperature reaches 165ºF, about 3 to 4 minutes per side. Transfer cooked chicken to a plate.

Add the remaining neutral oil to the pan and cook the rest of the chicken pieces in the same manner.

For grilling: Preheat an outdoor grill to medium-high heat (450°F to 500°F). Remove the chicken from the marinade and scrape off as much lemongrass as possible.

Clean grill grates, then arrange chicken in a single layer on the grill with space between pieces. Cover and cook until dark grill marks form, about 4 to 5 minutes.

Flip the chicken, cover again, and grill until grill marks appear on the other side and internal temperature reaches 165°F, about 3 to 5 minutes more. Remove from grill.

Allow the cooked chicken to rest for 5 minutes to retain juices. Slice into 1/2-inch pieces and serve with steamed white rice and broccoli if desired.