Bring a large pot of salted water to a boil and cook the rigatoni according to the package instructions. While the pasta cooks, prepare the pesto by combining the avocado, basil, parsley, garlic cloves, nutritional yeast, lemon juice and zest, broth, salt, and red chili flakes (if using) in a food processor or blender. Blend until smooth and well incorporated, adding more vegetable broth in small increments if a thinner consistency is desired. 

Once the pasta is done cooking, drain it and return it to the pot. Toss it over low heat with the prepared pesto until evenly coated and warmed through. 

Serve immediately and enjoy!