Beef Tossed with Wild Betel Leaf and Lemongrass: Bo Xao La Lot

2 tablespoons vegetable oil 1 lemongrass stalk, dry outer shell peeled off, and white part only, finely diced* 2 cloves garlic, finely diced 2 chiles, finely diced, plus 1 chile, finely sliced for garnish 12 ounces lean beef sirloin, thinly sliced 1 tablespoon fish sauce 1 tablespoon soy sauce 2 teaspoons sugar 15 betel leaves, roughly sliced* 2 sprigs fresh coriander (cilantro), for garnish Steamed jasmine rice, to serve *Can be found at specialty Asian markets and online.