In a large pot, bring salted water to a boil. Add the pasta and cook according to package instructions until al dente. Reserve 1 cup of pasta water then drain the pasta and set aside.
In a food processor, combine the ripe avocados, minced garlic, lemon juice, olive oil, fresh basil, and red pepper flakes (if using). Blend until smooth and creamy. If the mixture is too thick, add a splash of reserved pasta water to reach your desired consistency.
In a large bowl, combine the cooked pasta and the avocado sauce. Toss well to coat the pasta evenly. If the sauce is too thick, gradually add more reserved pasta water until it reaches your desired creaminess.
Taste and season with salt, pepper, and additional lemon juice if desired. If you’re using Parmesan cheese, mix it into the pasta now for added creaminess.
Divide the creamy avocado pasta into bowls, garnishing with extra fresh basil and a sprinkle of red pepper flakes for added color and flavor.