Melt 2 tablespoons butter in large pan over medium heat. Add flour and beat hard until you have a smooth paste (called a roux). Continue to beat until roux begins to have a golden color. Take off heat and gradually add stock, whisking constantly.
Place pan back over medium heat and simmer gently for 10 minutes, whisking frequently to ensure none of the sauce burns on bottom of pan. If sauce becomes too thick, whisk in a little more stock.
Add wine and continue simmering for 10 minutes, then take off the heat and whisk in cream and lemon juice. Taste for salt and pepper.
While sauce is simmering, melt 2 tablespoons butter in large frying pan until sizzling. Add chicken and fry for a few minutes until golden. Add mushrooms and fry for another 5 minutes or until chicken is cooked through.
To serve, mix sauce with chicken and mushrooms and sprinkle with some fresh tarragon or parsley.