Cook pasta according to package directions. Reserve ¾ cup pasta water before draining. If you want to serve cold, rinse under cold water.
While pasta cooks, make the sauce: blend avocado, garlic, lemon juice, olive oil, and salt in a blender or food processor until completely smooth. Taste — it should be bright, garlicky, and creamy. Add more lemon or salt if needed.
Thin with pasta water until pourable — start with 3 tbsp and add more until the sauce coats pasta nicely.
Toss pasta with avocado sauce immediately. The sauce will turn brown if it sits too long.