- 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.