I have been trying to find a recent document on installing Ubuntu (18.04 preferred) using mdadm RAID-1 mirrors and LUKS underneath mdadm. The main point of using LUKS under the mdadm layer is that encryption only happens once, whereas encrypting volumes twice carries further overhead and is a waste of resources.
Unfortunately I haven't quite found anything for UEFI systems.
This question does have a great answer for those who are not deploying LUKS: Install Ubuntu 18.04 desktop with RAID 1 and LVM on machine with UEFI BIOS
While I suppose that it should be as simple as creating a LUKS container immediately under the mdadm volume, and then LVM on the 'opened' container, I do not want to make assumptions about how easy or automated the process is with the initramfs. In the past there were issues when crypt_root was sitting inside a RAID array.
Hopefully others can share what they know through here.