Here is an exclusive look into why the file you are looking for is impossible to find, and the "backdoor" method that allows you to run the genuine operating system on standard PC hardware.
You can find the latest recovery images for "reven" (the codename for ChromeOS Flex) on Chromium Dash Direct Link (Latest Stable): You can often fetch the latest image via Google's download server Google Help 3. "Full" ChromeOS vs. ChromeOS Flex Install ChromeOS Flex: Fast, Secure OS for PCs & Macs
Brunch is not an operating system; it is a boot mechanism. It acts as a translator between generic PC hardware and the Chrome OS recovery image. Instead of downloading a fake ISO, you download the official Chrome OS recovery image (intended for Chromebooks) and "inject" the Brunch framework into it.