Wash your herbs and pick them off the main stalks. The smaller tender stalks are fine and actually add more flavour and bulk.
If you own a blender then make it work and add the ingredients roughly chopped into the blender along with the oil, seasoning, mustard, lemon zest and lemon juice. Blend till fairly smooth and you're done. Simple as that. And yes, taste it. Adjust if you want with more seasoning and lemon juice.
If you do this by hand and don’t own a blender, then you will chop the herbs, capers, anchovies, and gherkins as finely as you humanly can. Add the olive oil, lemon zest, lemon juice, mustard and seasoning. Mix and you're done. Obviously give it a taste to see if you want it more salty or sour.
Store in an airtight container in the fridge for up to a week or freeze for up to 6 months.