To perform out-of-band (OOB) remote management during the period of time between system restart and the initial load phase of the operating system you can simply redirect the BIOS/console to the server's serial port (as described below) then access the BIOS OOB through your serially connected console server.
Alternately for servers with BMC processors you can use Serial over LAN (SOL) to access the redirected BIOS OOB through the network connected console server (SOL requires firmware 2.2.0 or later). And for servers with service processors that support KVM (like the Dell DRAC) you can use the service processor management tool (like Dell Open Manager) to access the BIOS through the network connected console server
Refer to your BIOS vendor's documentation (AMI, Phoenix etc).
Also the majority of server manufacturers have been including this BIOS redirection capability with all new servers for some years e.g.:
With Windows systems when firmware redirection is coupled with Emergency Management Services, you can manage and monitor a server remotely as soon as the POST process begins and until the operating system components are initialized.