I’m not sure that anything is missing. The English translation for ‘une gousse de pois’ or 'une cosse de pois' is normally ‘a pod’, rather than ‘a shell’, and the WordReference dictionary gives the following example of its use :
Remove the peas from their pods, then cook in boiling water. Retirez les pois de leur cosse, puis faites-les bouillir
The word ‘shell’ can certainly be used in this context, but as a verb rather than a noun. An illustration of this use can be found in the article ‘Shell’, under the heading ‘Traductions supplémentaires’ :
Shell the peas, then cook them.
Écosse les petits pois, puis mets-les à cuire.