Blanch the asparagus for 3 minutes in boiling water. Remove with a slotted spoon and rinse with cold water.
Add pasta to the same water and cook until al dente.
In a frying pan, melt the butter over low heat. Add onion and cook five minutes or until softened. Add chicken and cook another minute. Increase heat. Add stock, cream, salt and pepper and tarragon. Bring to a boil and simmer 5 minutes or until chicken is done. Add asparagus and heat through.