Cook the lentils according to the instructions. Drain and set aside. In a medium pan, roast the walnuts without oil for about 2 minutes or until they're lightly golden. Put the walnuts and the cooked lentils in a food processor and process until chopped. Make sure to leave some texture!
In a medium pan, heat the olive oil over medium heat and sautée the onions for about 3 minutes. Then add the lentil walnut mixture and stir in the tomato paste. Cook for 2 minutes. Add the diced tomatoes and the spices (paprika powder, cumin, oregano, salt, and pepper).
Make the sour cream: Drain the cashews and put all ingredients in a food processor and process until smooth. Season with salt and pepper.
Fill the taco shells with the lentil walnut meat, tomato, avocado, kidney beans, corn, and lettuce and top with sour cream.