1 scallion, finely chopped
1/3 cup ketchup
¼ cup low-sodium soy sauce
3 tablespoons packed dark brown sugar
3 tablespoons fresh lime juice
1 tablespoon peanut oil
1 teaspoon grated fresh ginger
1 teaspoon minced garlic
¼ teaspoon crushed red pepper flakes
1½ pounds top round or beef tenderloin
¼ cup creamy peanut butter
In a small bowl whisk the marinade ingredients with ¼ cup water.
Cut the beef into ¾-inch cubes. Place the cubes in a large, resealable plastic bag and pour in the marinade. Press the air out of the bag and seal tightly. Turn the bag to distribute the marinade, place in a bowl, and refrigerate for 1 to 2 hours.
(If using bamboo skewers, soak in water for at least 30 minutes.) Remove the meat from the bag, reserving the marinade. Thread the meat onto skewers, leaving space between the cubes. Allow the meat to stand at room temperature for 15 to 30 minutes before grilling.
Prepare the grill for direct cooking over high heat (450° to 550°F). Brush the cooking grates clean. Grill the skewers over direct high heat, with the lid closed as much as possible, until cooked to your desired doneness, 3 to 5 minutes for medium rare, turning once.
Pour the reserved marinade into a small saucepan, bring to a boil over high heat, and boil for one full minute. Add the peanut butter, return to a boil, and whisk until the sauce thickens. Serve warm with the beef skewers.