Make the pesto: In a high power blender, add the spinach, basil, nuts, garlic, salt, pepper, Parmesan, and olive oil and blend until smooth. The key to a smooth, emulsified pesto lies in the blending process. The science behind emulsification is simple: it's about creating a stable mixture of oil and water-based ingredients. To prevent separation, add the basil gradually while the blender is running, ensuring the oil is slowly incorporated. This technique is a must, as it keeps the pesto emulsified. Our specific blending instructions (e.g., basil to oil ratio, speed of blending) prevent this.