1. For peanut sauce: Whisk peanut butter and hot water together in medium bowl. Stir in remaining ingredients; transfer to serving bowl and set aside.
2. For satay: Cut flank steak in half lengthwise and freeze for 30 minutes.
3. Slice each piece of steak across grain into 1/4-inch-thick strips. Combine fish sauce, oil, chili sauce, brown sugar, cilantro, garlic, and scallions in gallon-size zipper-lock plastic bag or large bowl. Add steak strips and toss to coat evenly. Refrigerate for 1 hour but no longer. Weave meat onto individual skewers—1 strip of meat if you’re using short skewers or 2 strips of meat for longer ones—and lay flat in shallow container.
4. To Grill: Turn all burners to high, close the lid, and heat the grill until very hot, about 15 minutes. Use grill brush to scrape cooking grate clean. Turn all burners to medium high.
5. Spread half of skewers out over hot cooking grate and grill, covered, until meat has cooked through and is lightly charred around edges, about 7 minutes, flipping them over halfway through grilling time. Transfer to serving platter and cover with foil to keep warm. Repeat with remaining skewers. Serve immediately with peanut sauce.