Combine basil, oil, nuts, cheese, garlic and salt in a food processor or chopper and pulse until finely chopped. Add a couple tablespoons of water and pulse until the pesto is as thin as you like. Taste it and add the lemon juice if you want.
Store pesto in a small covered container in the fridge. Pack it tightly then pour on a thin layer of more olive oil to keep it from oxidizing and turning brown. Pesto can also be frozen this way.