Mince the lemongrass very finely — or pulse the white inner core in a food processor. Woody fibers are the most common complaint in this dish, and fine mincing is the fix.

Whisk the lemongrass, garlic, fish sauce, sugar, soy sauce (if using), oil, and black pepper in a bowl. Add the chicken and turn to coat every piece.

Marinate covered in the refrigerator for at least 2 hours and up to overnight. Four hours is a good target — the flavor deepens without the meat turning soft.

Grilling (main method): preheat the grill to medium-high and oil the grates. Grill the thighs about 6 minutes per side, until well charred at the edges. If you brush on leftover marinade, do it only in the first minutes of grilling — never spoon raw-chicken marinade over finished meat. (To reuse marinade as a finishing sauce, boil it hard for at least 1 minute first; safest is to make a fresh batch.)

Pan-sear alternative: heat a heavy skillet over medium-high heat with a thin film of oil. Sear the thighs about 6 minutes per side without crowding the pan; work in batches. Crowding steams the meat instead of browning it.

Check doneness with an instant-read thermometer in the thickest part: chicken must reach 165°F (74°C) internal temperature (FoodSafety.gov).

Rest 5 minutes, slice, and serve over rice with cucumber and nuoc cham.