Peel and very finely chop the red onion, place in a bowl, cover with 4 tablespoons of red wine vinegar and add a very good pinch of sea salt.
Lower the eggs into a pan of vigorously simmering water and boil for 5½ minutes exactly.
Line up the asparagus, trim off the woody ends, and place in a colander above the eggs, covered, to steam while the eggs cook. Meanwhile, pick the tarragon leaves.
In a bowl, whisk the mustard with 6 tablespoons of extra virgin olive oil, adding it gradually, then pour in the soaking vinegar from the onion through a sieve, whisking constantly. Taste and season to perfection with black pepper.
When the time’s up on the eggs, divide the asparagus between your plates.
Refresh the eggs under cold water until cool enough to handle, then peel, halve and plate up, sprinkled with the tarragon leaves and onion.
Drizzle each plate with 1 tablespoon of dressing (save the rest for another day), season from a height with a pinch of pepper, and dig in.