Cook the asparagus in a large pan of
boiling water for 4-5 mins until tender.
Drain well, cool under running water,
then cut into finger-length pieces. Toss
together the tuna, beans, onion, capers
and asparagus in a large serving bowl.
Mix the oil, vinegar and tarragon
together, then pour over the salad.
Chill until ready to serve.