1. Preheat the oven to 200˚C/180˚C fan/gas mark 6. Bring a large pot of salted water to a boil and cook the pasta according to the package instructions. Drain the pasta and rinse under cold water until cooled, then set aside.
2. Melt the butter in a saucepan over a medium-low heat. Add the flour and cook, for 2-3 minutes, stirring constantly until combined.
3. Gradually add the chicken stock, stirring to combine. Stir in the cream.
4. Add the Parmesan, mustard and tarragon. Season to taste with salt and black pepper.
5. Cook for 8-10 minutes, until the sauce thickens slightly. Add the chicken and pasta and stir to combine thoroughly. Transfer to an oven-proof casserole dish.
6. Sprinkle breadcrumbs evenly over the top and bake for 20-25 minutes until the top is golden brown and the edges are bubbling.
7. Serve hot with a simple green salad.

Per serving: 455kcals, 22g fat (5.1g saturated), 2.7g carbs, 1.7g sugars, 58.5g protein, 0.8g fibre, 0.757g sodium