Add walnuts and seasonings to a food processor and process until mixture is kind of “meaty”.
Put the sour cream ingredients into a small bowl and use a fork to combine. Add more lime juice and water to achieve the consistency you want. Slowly add the salt to balance the bitterness of the lime.
Place a small spoonful of the cream and wipe it through the center of the shell. Fill taco shells with walnut mixture and top with toppings of choice. Drizzle a small amount of the cream and finish with cojita.