Step 1 Puree the garlic and salt using a mortar and pestle or in a blender. Add the walnut halves and grind to a thick paste. Add the vinegar and puree a little more. The paste will loosen and smooth out.
Step 2 Beat in olive oil just until the paste holds its consistency. When you see oil start to separate, stop.
Step 3Stir in the basil leaves and season to taste with more salt if necessary. The pesto can be prepared up to 2 hours before serving. Stir vigorously before serving.