Heat half the oil in a skillet over moderate heat.
2 tbsp olive oil
Season the chicken with salt and pepper and brown on both sides.
6 large chicken breast halves, salt, ground black pepper
Transfer to a greased baking dish.
In the same skillet, heat the remaining oil and sauté the scallions, garlic, and mushrooms for 5 minutes.
2 tbsp olive oil, 4 large scallions, 2-4 cloves garlic, 8 oz mushrooms
Add to the baking dish. In the same skillet add the stock, wine, tarragon, salt and pepper and bring to a boil, stirring constantly.
1/2 cup chicken stock, 1/4 cup dry white wine, 2 tbsp tarragon, salt, ground black pepper
Remove from the heat.
Mix the flour and sour cream in a small bowl and whisk into the liquid until smooth.
2 tbsp all-purpose flour, 1/2 cup sour cream
Pour over the chicken and bake in a preheated 350F oven until the chicken is cooked through, about 35 minutes.
Serve the tarragon chicken and mushroom casserole with the mushrooms spooned over the chicken and garnished with a tarragon sprig if desired on a bed of hot bed of white rice or new potatoes and seasonal greens.