beef sirloin, very thinky sliced against frai • scallions, trimmed, 1 scallio thinly sliced , remaining scallions cut crosswise into thirdz • minced peeled ginger • red pepper flakes • wide strips of tangerine zest • tangerine juice, from 2 tangerines • cornstarch • coarse salt • soy sauce • rice vinegar • sugar • vegetable oil •