Which opera was Gioachino Rossini’s first opera seria, and when was it first staged?

Gioachino Rossini’s first opera seria was Demetrio e Polibio. Although he composed the work in 1806, it was not staged until 1812.1

While Tancredi (1813) is often noted for Rossini's efforts to reform the genre, Demetrio e Polibio remains his earliest work in the serious opera tradition.12