Josh Weissman has a good looking Katsu curry sauce recipe he makes from scratch.
https://www.joshuaweissman.com/post/perfect-homemade-katsu-curry
You could ditch the MSG and still be ok.
Yeah, that sauce is an overcomplicated faff.
Make-your-own is a 10 minute job (ignoring the katsu for a minute).
(YMMV, IANAC, Personal Preference Notwithstanding, Batteries Not Included)
@NotThamesWater started
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Josh Weissman has a good looking Katsu curry sauce recipe he makes from scratch.
https://www.joshuaweissman.com/post/perfect-homemade-katsu-curry
You could ditch the MSG and still be ok.