Q: How do you get Windows 7 via parallels to recognise Thunderbolt to Ethenet adapter?

It's parallel's responsibility. Update parallel tools.

Indeed, it is the hypervisor and its support drivers to map the physical hardware to the logical hardware it presents to the guest systems. Besides updating the product to the newest version and ensuring that you have installed the Extensions/Tools/whateverParallelscallsit on the VM. AND loudly complain with that maker in case that doesn't  work.

That being said, in older days and when using the competition VMware, we used to improve functionality and performance by installing the set of Apple drivers that come with BootCamp. In addition to the Extensions/Tools/etc.

Actually it is not up to windwos 7 or parallels. The vm should pick up all installed hardware from the host os and as to a network connection the vm os only cares if there is a network connection on the host and then that is mapped to the virtual network adapter in the virtual machine.

So if you want the virtual machine os to see and use the thunderbolt to ethernet adapter as its active network adapter connection then that network connection has to be active and is being used by the host os before you start the virtual machine os.

I don't use parallels but I have used both VMware fusion and Oracle virtualbox and they are both the same with the VM's active netowrk connection. Whichever the host is using as its active network connection the vm uses, So If I am using the wifi on os x when I start the vm it uses that wifi connection but to the vm it looks like a ethernet connection. If the host is using the physical ethernet port, the system is wired to my router, then the vm maps that as it active network connection. Again to the vm it looks like a standard ethernet connection. In virtual box you can have multiple network adapters and you can map them however you choose. Not sure how parallels work in this respect.