Pre-heat the oven to 190°C.
Make a few diagonal slashes in each chicken breast. Mash together the butter, half the chopped tarragon, the garlic and some seasoning until smooth. Stuff this butter into the slashes of the chicken breast even.
Arrange the chicken breasts on a lined baking tray. Bake for 20-25 minutes until the chicken is cooked through and a little springy to the touch, with no pink showing.
Meanwhile, steam the beans in a steamer set over a pan of gently simmering water for 4-5 minutes until just tender. Transfer to a mixing bowl and dress with the wholegrain mustard and some seasoning. Arrange the beans on serving plates and lay the chicken breast on top. Garnish with a sprig of tarragon and serve immediately.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home and view the full Food safety at home video.