How do you mount a fresh LUN disk to your / ?
Do you use ext3 or NFS for this ?
Neither nor. You use ISCSI or the corresponding SAN protocols for Fibre channel. You then see the disc like a local (unformatted) disc and it is up to you to format it however you like it.
A LUN disc basically is a physical disc (for your operating system) that just happens not to be physically attached to the computer.
As such, mounting a fresh LUN to / is not possible - it is not formatted.
By "fresh LUN" it sounds like it's a newly created and attached block device with no file-system already on it.
First, you can verify that the device is connected to your host and that it has a device name that you can use to reference it. There are several different ways, for example using 'fdisk -l' or possibly 'lsscsi' if it's an iSCSI device.
Once you have the correct device name, say /dev/sdd1, then you can create your favorite file-system on it as any other local block device or disk.