For the casserole: Heat a few inches of water to a low boil, add salt and the green beans and cook 3 to 4 minutes to tender-crisp, drain
Heat the EVOO in a large, deep skillet over medium to medium-high heat, add the mushrooms and lightly brown them, 7 to 8 minutes. Add the butter, shallots and garlic, season with salt, pepper and thyme, and stir 2 to 3 minutes more. Add the flour, stir a minute, add the wine, stir in the stock and thicken. Add the cream
and reduce the heat to simmer. Add the green beans
and chicken and thicken a bit; stir in the tarragon
Transfer into a casserole dish
and cool. Cover and refrigerate for a make-ahead meal.
To reheat: Bring back to room temp. Bake at 375 degrees F until bubbly, 50 to 60 minutes; top with homemade or store-bought Fried Onions.