ASUS Prime Z790-P, an Intel Z790 LGA 1700 ATX motherboard with PCIe® 5.0, three M.2 slots, 14+1 DrMOS, DDR5, Realtek 2.5 Gb LAN, HDMI®, DP, USB 3.2 Gen 2x2 Type-C®, front USB 3.2 Gen 2 Type-C®, Thunderbolt™ (USB4®) support, Aura Sync
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then you can't flash the BIOS without a screen or any form of a display output from the platform. You will need to drop in a processor preferably from the 13th Gen Intel processor that's not an F SKU so you can work with the iGPU, one single stick of ram in slot A2 and then power the system with a PSU. You won't need storage or a discrete GPU but you will need a spare system that's able to connect tot he internet to download the latest BIOS version off of Asus's support site.
Had you a motherboard with the BIOS FlashBack button on the rear, then you merely needed a PSU, no need for a display.