Resources on how to write a UEFI application from scratch?

I'm interested in building a toy OS, and the modern way of building one seems to be using UEFI. There are a few tutorials for writing a simple UEFI OS, but most of them use either GNU-EFI or EDK2, and I'd like to build one from scratch using only a C compiler, assembler, and linker. I tried getting clang to produce a PE32+ executable (I'm on Linux), but was unsuccessful. Any tutorials or places to find good documentation on this? (wiki.osdev.org has much more about legacy boot than UEFI)