Bring a large pot of water to a boil. Salt the water, add the pasta, and cook according to the package instructions until al dente. Reserve but do not drain the pasta.
On a cutting board, combine the fresh coriander (cilantro), walnuts, and lime zest and coarsely chop. Transfer to a small bowl, stir to combine well, and set aside.
In a blender cup or tall jar, combine the avocado, shallot, lime juice, and a pinch of salt and purée with a hand blender.
Add a small ladleful of the pasta cooking water and the olive oil to the avocado purée and blend briefly to combine, then stir in the chilli.
Drain the pasta and transfer it to a serving bowl. Add the avocado sauce and toss to coat, then sprinkle with the cilantro-walnut mixture and serve.