Grilled Jumbo Shrimp

With a paring knife, make a 1⁄4'' incision, through the shell, along the back of jumbo shrimp. Remove and discard veins, rinse shrimp, pat dry, and place in a bowl. Remove outer leaves from lemongrass, trim ends, and thinly slice inner stalk. Combine lemongrass, sugar, and Vietnamese fish sauce in a mortar and grind to a coarse paste. Spoon mixture over shrimp, mix well, cover with plastic wrap, and refrigerate for 2 hours. Remove shrimp from refrigerator and mix in oil. Heat a cast-iron grill pan over medium-high heat. Cook shrimp until pink and firm, 1-2 minutes per side. Serve with white rice.