How to use fastboot to manually force flash a factory image? (locked out of recovery mode)

I tried to root my oneplus 2, which had already been rooted before receiving an OTA update. I failed to completely flash TWRP, but it locked me out of recovery mode, I could not boot up TWRP, and the menu factory reset buttons are grayed out.

Oxygen 3.0.2

Phone works normally most of the time, however i am unable to install updates.

I use a mac. How can i use fastboot to force my phone to flash the factory image?

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