Fast startup has nothing at all to do with the BIOS settings. That is completely related to Windows fast startup/hybrid sleep routine, and is not related to the "fast boot" in the BIOS. And, it's a pile of crap. I recommend ALWAYS disabling hybrid sleep/fast startup by disabling hibernation, for anything that isn't strictly a tablet or notebook, and I USUALLY disable it on notebooks as well.
If your B450 Mortar isn't showing the setting in the BIOS then it's likely/possible that your configuration is legacy, not UEFI. If the configuration and Windows installation are not FULL UEFI then that setting may not be visible. If it is a full GPT partition and UEFI configuration and installation, and the setting is still not there, then it is...