I tried first by installing ROM Manager (126.96.36.199, latest version at the time I am writing). Under Recovery Setup, I found that I supposedly had already installed both ClockwordMod Recovery and TWRP. I honestly don't recall installing either one, but the options were available. I tried to "Reboot into Recovery" first with CWM, then with TWRP - both times, after the initial LG logo, I was greeted by the broken android with the red triangle. I also tried to reinstall CWM by choosing the corresponding option "Install or Update Recovery"), but the result was always the same. Each time, I was able to restart the phone, which is still working fine.
I then decided to flash a new CWM. I downloaded CWM Touch, then used Flashify to flash the new image and reboot the phone. This time, the boot process stopped after the LG logo appeared, but an error appeared right above it. I repeated the process two more times (once with CWM Touch and once with the default CWM), and the message that I read was something like:
Secure booting error Error: Boot Certification Verify!