First cut away the woody ends of the asparagus spears. Rinse with cold water and cut into pieces about 4 cm long.
Cut the lemongrass into very fine rings. Heat oil in a pan and sauté asparagus pieces in it. Season with lemon grass, finely chopped tarragon, salt and pepper and pour in the whipping cream. Let it boil down slightly.
Bring plenty of salted water to the boil with a few drops of oil and cook the noodles in it until al dente. Strain and toss in a little butter.
Put the farfalle into deep plates and serve with the asparagus sauce.