Place steak in freezer 30 minutes to firm; slice into 1/4" thick strips. In shallow glass dish, combine soy sauce, sherry, sesame oil, onion, garlic, sugar, ginger and 1/2 tsp of the red pepper flakes. Add beef strips; turn to coat with marinade. Cover and refrigerate 2 to 4 hours. Soak twenty-four 8" bamboo skewers in water for 20 minutes. Drain beef; reserve 2 Tbl marinade. Thread beef strips, accordion-style, on skewers. Meanwhile, in small saucepan, combine reserved marinade, remaining 1/2 tsp red pepper flakes, the peanut butter and water. Heat over low heat 8 to 10 minutes or until sauce is thick and warm (add more water if necessary). Grill kabobs, on uncovered grill, over medium-hot briquets 2 minutes. Turn and cook 2 miutes longer. Serve beef strips with sauce. NOTE: Assembled kabobs may be refrigerated, covered, 1 to 2 hours before grilling.