Preheat the oven to 200C (390F).
Put the pasta on to cook. When ready, drain, rinse and set aside. While the pasta is cooking, make the sauce:
Melt the butter in a saucepan, add the flour and cook for a couple of minutes, stirring constantly. Keep the heat low to medium, to avoid browning.
Add the chicken stock and cream and stir thoroughly.
Add the cheese, mustard and tarragon. Taste and adjust seasoning.
Cook for a further 10 minutes, until the sauce thickens a little.
Once the sauce is ready, add the chicken and drained pasta and stir thoroughly.
Transfer into an oven-proof casserole dish.
Sprinkle breadcrumbs evenly over the surface.
Bake for 20-25 minutes until the crumbs on top are golden brown.