Boil the pasta. Bring a large pot of salted water to a rolling boil.
Cook the pasta until just al dente according to package directions. Before draining, reserve at least 1 cup of the starchy cooking water.
Prep the sauce base. While the pasta cooks, add avocado, garlic, lemon zest, 2 tablespoons lemon juice, basil, red pepper flakes, 1 teaspoon salt, and a few grinds of pepper to a blender or food processor.
Blend it smooth. With the machine running, stream in the olive oil. Add 2–3 tablespoons of hot pasta water and blend until silky.
Taste and adjust with more lemon, salt, or pepper. The sauce should be thick but pourable.
Toss and adjust. Return drained pasta to the warm pot. Add the avocado sauce and a splash of hot pasta water.
Toss until every strand is glossy. Add more water a little at a time until it’s creamy and well-coated.
Finish and serve. Top with extra basil, a drizzle of olive oil, and optional Parmesan, nuts, or cherry tomatoes. Serve immediately while the pasta is warm and the sauce is bright.