Cut the avocado into 1cm cubes and thinly slice the onion.

Boil water in a pot, add salt (1/2 tbsp), and cook the penne according to package instructions. Reserve 50ml of pasta water and drain the rest.

Heat butter in a frying pan over medium heat, add the onion, and sauté until softened. Add flour and cook until no longer powdery.

Add soy milk, reserved pasta water, Parmesan cheese, salt, and pepper. Stir and cook until the sauce thickens. Add the penne and avocado, mixing well.