Put the garlic, shallot, preserved lemon, salt and lemon juice in a small processor or blender. Pulse until everything is chopped and leave for 5 minutes – this will take the harsh edges off the shallot and garlic.
Add the parsley and coriander to the processor and pulse until chopped and a thick paste forms.
Now add the oil, just a tablespoon at a time, scraping down between pulses. You may not need the entire ¼ cup at all, depending on how much leafage you have used. Add only enough to make the mixture come together as a thick but just-pourable sauce.
Store in a screw-top jar in the fridge until needed, letting it come back to room temperature before serving.
