Toast the walnuts. Bring a small skillet on the stove to medium-low heat. Stir for about 3-5 minutes until walnuts are lightly browned and fragrant.
Blend the spinach basil pesto. Add fresh basil, spinach, toasted walnuts, olive oil, lemon zest and juice, salt, pepper, garlic, red pepper flakes, and ¼ cup water to a food processor.
Puree pesto ingredients until smooth.
Do I really need a food processor? You could use a mortar and pestle or other kinds of blending appliances if you do not have a processor. That being said, with this spinach and basil pesto recipe in particular, it really is much easier to use a good food processor with a little more oomph.
Can I use canned or frozen veggies? Technically, yes, but I would not recommend! Not only will you taste the difference in your pesto, but fresher ingredients pack more nutrients than old or cooked ingredients. Avoid the urge to buy pre-made or processed foods as much as possible.