To make the sauce, put the mint, garlic, preserved lemons and juice, and half the oil into a bowl and patiently blitz with an immersion blender. When most of the leaves have been incorporated, pour in the remaining oil and blitz again, until you have a deep emerald, emulsified sauce. Taste to see if you want to add salt, though I find the preserved lemons give all the salinity you need, and I am an enthusiastic salter.