In a medium bowl, combine the finely chopped lemongrass, minced garlic, and grated ginger. Add in the fish sauce, sugar, and optional chili for a hint of spice. Mix well until the sugar dissolves and the ingredients are fully incorporated.

Place the chicken pieces in the marinade, ensuring each piece is thoroughly coated. Cover the bowl with plastic wrap or transfer to a zip-top bag and refrigerate for at least 30 minutes, ideally overnight.

Heat a tablespoon of vegetable oil in a large pan over medium-high heat. Swirl the oil in the pan until it shimmers.

Add the marinated chicken pieces to the pan in a single layer. Cook for about 5-7 minutes on each side, until the chicken is golden brown and cooked through.

Once cooked, remove the chicken from the pan and let it rest for a few minutes. Slice the chicken and serve it over a bed of steamed jasmine rice.