Halve the flank steak lengthwise, then slice on slight angle against the grain into 1/4" thick slices.
For the basting sauce: Whisk all ingredients together in a bowl. Reserve one third of sauce in separate bowl. (Use reserved sauce to apply to raw beef.
FOR THE BEEF: Whisk oil, sugar and fish sauce together in a medium bowl. Toss beef with marinade and let stand at room temperature for 30 minutes. Weave the beef onto metal skewers, 2 pieces per skewer, leaving 1 1/2 inches at top and bottom of skewers exposed.
4 For gas grill- Turn all burners to high, cover, and heat grill until hot, about 15 minutes. Leave all burners on high.
Clean and oil cooking grate. Place beef skewers on grill (directly over coals if using charcoal) perpendicular to grate. Brush meat with on-third basting sauce (portion reserved for raw meat) and cook (Covered if using gas) until browned, about 3 minutes. Flip skewers, brush with half of remaining basting sauce and cook 1 minute longer. Transfer to large platter and serve with peanut sauce.