Trim any excess fat from the sirloin and slice the steak into small, thin pieces about 3/4" (2 cm) square and 1/4" (5 mm) thick.
Prepare the marinade by combining minced lemongrass, minced shallot, minced garlic, fish sauce, sugar, sweet soy sauce, sesame oil, salt, and black pepper in a mixing bowl.
Add the sliced beef to the marinade and toss thoroughly to coat; marinate in the refrigerator for at least 1 hour.
While the meat marinates, soak wooden skewers in water for at least 30 minutes to prevent burning.
Thread the marinated beef pieces onto the soaked skewers, leaving a little space between pieces for even cooking.
Preheat a grill or grill pan to medium-high heat and grill the skewers 2–3 minutes per side, or until the meat reaches your desired doneness.
To make the sautéed scallion garnish, heat a non-stick skillet with 2 teaspoons oil until hot, add the chopped scallions, season with a pinch of salt, and sauté about 1 minute.
Remove skewers from the grill and garnish with crushed roasted peanuts and the sautéed scallions before serving.