What are the methods for gaining Bezels in battle?

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From what I understand, you gain Bezels from killing enemies and shooting off parts, but when I performed a Heroic Action while fighting a bunch of Fat Gangsters (basic jump over them during my run doing direct), I didn't kill any, but regained a Bezel.
When I did it again, (same action but doing scrap damage on a different Fat Gangster), I didn't gain any.
In both cases I didn't see any parts fall off or anything like that, nor did they become airborne (as I was shooting from above).
What are all the methods of regaining lost/spent Bezels?
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