Place garlic and salt in a mortar; smash and mash with the pestle until it becomes a paste, 5 to 8 minutes. Add walnuts; mash walnuts into a paste until you reach your desired texture (coarse or smoother), another 5 to 8 minutes. Add parsley and mash to break up and blend with the walnut/garlic mixture, about 4 minutes. Add lemon juice and olive oil; stir, mash, and emulsify liquids into the mixture until it becomes very thick and well blended, 8 to 10 minutes. Taste and adjust seasonings. If mixture seems too thick, whisk in a tablespoon of water, or as needed.