Surf instructor Sarah Brady -- JH's former flame of about a year from back in 2021 -- is dragging the actor publicly on her Instagram account ... and she's not holding anything back in her repudiation of him as an alleged "misogynist narcissist," as she puts it herself.

Sarah has thrown up several text messages that she attributes to Jonah, and if they're to be believed ... it appears he was often concerned/upset about her posting photos of herself.

You can hop onto her IG Story for yourself to see the full characterization she's putting out there -- but bottom line ... it doesn't make Jonah look good at all. Sarah suggests Jonah was manipulative and controlling in their relationship ... and hurls even more allegations.

A lot of the alleged texts she posted appear to show Jonah as constantly bickering at her over a number of things -- whether it's photos of herself surfing, how she handles herself in social situations or even what a therapist she was seeing apparently thought of him.

She also blasts his understanding of "boundaries" ... and suggests he would often gaslight her to keep her under his thumb. BTW, the two of them would hit public events together -- and were photographed as a pair a handful of times.

That's what makes his even uglier -- now, she's even throwing around the word "abusive" to describe their dynamic ... although in a screenshot of a DM she posted, she makes it clear that the alleged abuse she claims she endured with Jonah was emotional, not physical.

Now, since Jonah and Sarah parted ways ... he has moved on with a woman named Olivia Millar, with whom he now shares a child. The couple welcomed a baby this past May, although the sex and name have yet to be revealed. Welp, Sarah's got thoughts on that too.

She says she hopes her ex -- obviously referring to JH -- has a girl, so that he can learn to become a true feminist. Sarah thinks the fact he calls himself a feminist now is laughable.

One last dig she takes at Jonah stands in a photo she posted on her main timeline. It's a black and white shot of her, and it's one she claims Jonah was not comfortable with.

Sarah says, "reviving a pic I took down by request of a misogynist narcissist." In other words, she's alleging he actually asked her to take it down and leave it down, and that she did.

Jonah doesn't actively use social media, and does not appear to have responded to everything Sarah's claiming about him (a fair amount, no doubt). We've tried reaching out to his camp a couple different times now ... but so far, haven't heard anything back.