Trim the asparagus ends and peel the tough parts with a peeler. Cut into 3cm pieces. Thinly slice the onion vertically.

Heat butter in a frying pan over medium heat. Add onion and asparagus, cooking until softened. Add clams and white wine, cover, and cook until clam shells open.

Boil water in a pot, add salt (not listed in ingredients), and cook spaghetti 1 minute less than package instructions. Drain.