Add the walnuts and olive oil to a blender or food processor and blend until mostly smooth. Add the garlic and blend until smooth. Add the basil leaves, small amounts may be necessary depending on your blender/food processor. Add the lemon juice and salt. Processor until it reaches your preferred consistency. I tend to leave mine a bit chunky. Fill an ice cube tray with the pesto. One batch should fill one ice cube tray (in the picture above I reserved a bit for my dinner that night so it didn't quite fill a tray). Freeze until solid and then transfer into a freezer container or bag. Keep in your freezer for up to a year and use as you need it!