Explain why the verb ‘mettre’ in its past tense form as it appears in the following sentence “Il a décroché la casserole et l’a mise sur la table” has a feminine ending or terminaison (despite the auxiliary’s verb ‘avoir’’s neutrality)?

Generally the past participle doesn’t agree with any noun when the auxiliary verb ‘avoir’ is used. For example: ‘Elles ont acheté des bottes’ despite the subject ‘elles’ and the direct object ‘bottes’ being feminine and plural. 

Notwithstanding, when the direct object precedes the past participle verb, the latter agrees with it: ‘J’ai rangé les bottes qu’elles avaient achetées’. 

In the majority of cases, the conjunction ‘que’ which refers back to the direct object prompts this change in verb/noun agreement. 

In the initial question, ‘l’a mise’ refers back to the casserole.