Combine basil, spinach, walnuts, lemon juice (if using), and minced garlic in a food processor bowl.

Process the ingredients until finely chopped.

With the processor running on high, slowly drizzle in the olive oil through the feed chute.

Continue processing until the pesto reaches your desired consistency.

Add more olive oil, 1-2 tablespoons at a time, if needed to achieve a smoother consistency.

Taste and adjust seasonings as necessary (e.g., more lemon juice, salt, or garlic).