How do I upgrade my rooted Nexus One using OTA updates? I have a Nexus One that I rooted years ago. It's running "stock" Froyo. I get offered OTA updates to Gingerbread, and I'd like to do the update.

When I choose the update, it reboots the phone, which brings me to the ClockworkMod bootloader. Now what?

Can I make this work somehow?
OTA updates can only be applied to a completely stock, non-rooted phones, using the stock recovery.

If you really want a completely stock Gingerbread, you will need to un-root your Nexus One and revert to Froyo's stock recovery.  See instructions here.

Alternatively, you can flash one of the custom Gingerbread ROMs.  Some are very close to "stock" (only pre-rooted) while others like CyanogenMod offer other tweaks on top of the original.