Never ever dream of making this in a mortar and pestle. It’s incredibly tedious, and to my mind only succeeds in over-oxidising the basil, turning it an unattractive khaki colour. The quantity made here requires an enormous amount of basil, and I recommend you make it in summer when it is relatively cheap. You will also need a couple of clean screw-top jars to keep it in. The best way of sterilising a jar these days is to run it through a dishwasher at high heat.