I don't think you can.
You can restore your data from TM, but I don't think you can restore the OS. It is the probably the worse thing about TM.
Just a tip for the future, before you go to update to a new OS, clone the old one. I have a few external HDDs that is partitioned to fit a bunch of boot drives on it. Before upgrading, or to make an occasional back up, I just use CCC to clone to one of the drives.
I also use the drives to store bootable installers of the MacOS versions. It is really easy, although it involves using the Terminal app, but still really, really easy.
But, back to your problem, I am not sure if you can make a bootable installer if you are using a newer OS. I think you can make a current OS installer, and newer OS installers, but not older.
Anyone else know?
Do you have any access to Macs with older MacOS?