Sauce: Remove avocado from skin and place in a small bowl. Mash with a fork until fairly smooth - it doesn't have to be perfect. Stir in lime juice, mayonnaise, onion, almonds, salt and pepper. Blend well.
Cook pasta according to package directions.
Drain and return to pot. Drizzle with melted butter. Mix to coat the pasta then serve up each individual portion.
Spoon on a generous dollop of avocado sauce and gently mix. (The heat from the pasta will "melt" the sauce much like any other pesto), sprinkle with parmesan and enjoy.