This assumes AMDGPU drivers are already installed and functioning in Steam and/or Lutris; just probably not to the full potential.

With AMDGPU installed, there should be some ICD JSON files located at /usr/share/vulkan/icd.d/:

ls /usr/share/vulkan/icd.d/
amd_pro_icd32.json  amd_pro_icd64.json  radeon_icd.i686.json  radeon_icd.x86_64.json

I’ve found that the radeon_icd files enable DX-12 in Steam, but the other files may need to be tried.

Modify your launcher shortcut to Steam and set the execute command to the following (or whichever ICD file works):

/bin/bash -c "VK_ICD_FILENAMES=/usr/share/vulkan/icd.d/radeon_icd.x86_64.json steam %U"

Not every game in Steam will work with this enabled, so making a secondary application shortcut running the default ‘steam %U‘ will allow for switching between the two Steam configurations.

In Lutris, right-click your installed game or launcher and go into Configure. Go to ‘System Options’, scroll down to ‘Environment variables’ and add a new entry:

VK_ADD_DRIVER_FILES = /usr/share/vulkan/icd.d/radeon_icd.x86_64.json

Both of these fixes should give better overall performance and also allow games to use DirectX 12.