I have seen articles like 秘密のとんかつレシピ. This would mean that the prior word is defining the latter, it's the secret Tonkatsu Recipe. Is this true? Thanks in advance!
It's not defining, it's modifying or clarifying.
Think about this phrase:
What kind of movies? Japanese movies. What kind of tonkatsu recipes? Secret tonkatsu recipes.
A lot of different relationships between A and B can be written as AのB in Japanese, and you understand the correct meaning in context. For example, ピカソの絵 could mean paintings of Picasso, paintings created by Picasso, or paintings belonging to Picasso.