You an try `fastboot flash boot_a <image_filename.img>` or `fastboot flash boot_b <image_filename.img>`. The `boot_a` is the name of our recovery boot partition. If your FP was rooted several times this my result in a non factory partition name ...
With `fastboot getvar current-slot` you can find out whether your suffix should be `_a` or `_b`. Find more possible names at https://forum.fairphone.com/t/find-out-boot-partitions-name/77171
