A retortable container package includes a container and a composite closure. The container includes a 60 to 140 taper formed on an inboard side of the finish portion. A raised channel of the closure's insert disk includes a substantially flat top portion and an inner wall sloped at approximately the same angle as the taper angle. A sealant in the channel is deformed upon application of the closure onto the container such that the sealant forms a seal between the closure and each of (i) an outer wall of the finish portion proximate the lip, (ii) the lip, and (iii) the taper. Upon retorting, the seal between the container's taper and the channel inner wall is enhanced by the contraction of the finish portion diameter. A corresponding method includes an application step.