In a dry small heavy skillet cook sesame seeds over moderate heat, stirring, until golden brown, about 4 minutes, and transfer to a bowl. In a 5-quart saucepan bring 4 quarts salted water to a boil. Add orzo and cook, stirring occasionally, until al dente. Drain orzo in a colander and rinse under cold water. Drain orzo well and add sesame seeds. In an 8-inch non-stick skillet heat oil over high heat until hot but not smoking and saute scallions, stirring occasionally, until lightly charred and tender, about 3 to 4 minutes. Stir scallion mixture into orzo and season with salt and pepper.