Lemongrass Beef Skewers

Name: Lemongrass Beef Skewers
Ingredients: 3 lemongrass stalks, bottom 4 inches only, minced; 3 tablespoons plus 1 1/2 teaspoons fish sauce; 2 tablespoons plus 1 teaspoon packed light-brown sugar; 1 garlic clove, minced; 1 tablespoon vegetable oil; 1 pound boneless sirloin, trimmed of excess fat; 1/4 cup roasted salted peanuts (about 1 ounce), crushed; 1/2 English cucumber, thinly sliced on the diagonal (optional); Fresh mint and basil sprigs, for serving; Bibb or Boston lettuce leaves, for serving; Lime wedges, for squeezing