How to Make It
In a food processor, pulse the lemongrass until finely chopped. Add the scallions, garlic, jalapeño and sugar and pulse until finely chopped. With the machine on, add the 1/4 cup of vegetable oil in a steady stream and process to a fine paste. Season the paste with salt and pepper.
Using a small, sharp knife, make 1/2-inch-deep slashes into the chicken and rub the paste all over, working it into the slashes. Marinate the chicken for 15 minutes at room temperature or refrigerate overnight.
Light a grill. Brush the chicken with oil, season with salt and pepper and grill over a medium-hot fire, turning occasionally, until browned and cooked through, about 15 minutes.