Boil the pasta. Bring a large pot of salted water to a rolling boil.
Add pasta and cook until al dente. Scoop out about 1 cup of pasta water before draining.
Prep the sauce base. While the pasta cooks, add avocado flesh, garlic, lemon zest, 2 tablespoons lemon juice, olive oil, a pinch of salt, and black pepper to a blender or food processor.
Blend until creamy. Start blending, adding splashes of warm pasta water until the sauce is smooth and pourable. Aim for a texture like thin Alfredo.
Taste and adjust lemon, salt, and pepper.
Add herbs. Toss in basil or parsley and pulse just a few times to keep the sauce bright green and fresh, not overworked.
Toss with pasta. Return drained pasta to the warm pot. Pour in the avocado sauce and toss over low heat for 30–60 seconds, loosening with more pasta water as needed until silky and glossy.
Finish and serve. Add a pinch of red pepper flakes, extra lemon zest, and a shower of Parmesan if using. Fold in any add-ins now—like halved cherry tomatoes or handfuls of arugula—just to warm through.
Serve immediately.