Slice chicken in half lengthways so that you have 4 thinner pieces.
Season with salt then fry in a splash of olive oil in a very hot pan for roughly 2 minutes each side then remove from the pan and set aside.
In the same hot pan, add the mushrooms and fry on a really high heat to get some colour.
Reduce the heat & add the onions and sweat for 10 minutes.
Add the butter into the pan with the garlic and half of the chopped tarragon and fry for a few minutes.
Add the tomato puree into the pan, mix through and let that cook off for a few minutes.
Sprinkle in the flour and combine with the other ingredients to create a thick paste.
Pour in the stock in small instalments stirring in between each batch until well mixed.
Bring up to a simmer for 2-3 minutes.
With the heat on low, add the single cream & both mustards and stir through.
Add the chicken back into the pan with a good grinding of black pepper and the second half of the tarragon.
Serve generously into bowls with fluffy white rice.
