Peel the asparagus bases with a swivel peeler and trim the woody ends. Blanch in boiling salted water for 1½-2 mins until just tender. Drain and refresh under cold running water.
Trim the base of the lettuce leaves and arrange on a large shallow bowl or platter. Scatter over the asparagus spears and red onion, season well and drizzle with vinaigrette just before serving.
Here’s my quick way to prepare asparagus. Simply bend each spear and snap where you feel it naturally give, breaking off the woody end.