La fondue suisse / délicieux

Question

Hello. Why doesn’t délicieux agree with la fondue suisse? I heard the correct pronunciation in the dictation, but I thought I must have been mistaken, so I wrote délicieuse.

Maarten Kamp · Accepted Answer

Eileen,  ‘c’était vraiment délicieux ‘.  
The indefinite demonstrative pronoun is the subject of the verb ‘être’ , and is the word being ‘ described/ modified ‘ by the adjective. 
In this construct ( regardless of tense used )  stand-alone adjectives following ‘ c’est ‘ are always masculine.
 https://progress.lawlessfrench.com/revision/grammar/adjectives-following-cest-are-always-masculine 
https://www.lawlessfrench.com/grammar/indefinite-demonstrative-pronouns/