Chicken with Mushrooms, Tarragon, and Mustard

4 boneless, skinless chicken thighs
salt and pepper
1/4 c butter
1/4 c white wine or sherry
1/2 of a large onion, chopped
2-4 cloves of garlic, minced
12 large mushrooms (about a pound), sliced
1 c heavy cream
1 T mustard (I had spicy brown this time, but Dijon might be better)
2 t dry tarragon

Season chicken with salt and pepper.
Melt the butter in a dutch oven.
Brown the chicken on both sides, then cover and cook over medium heat, turning occasionally, for 15 minutes.
Remove the chicken from the pan, and set aside.
De-glaze the pan with the wine.
Saute the onion, garlic, and mushrooms until they are mostly cooked.
Add the cream, mustard, and tarragon.  Cook, stirring occasionally, for about 5 minutes, until the sauce starts to thicken.
Return the chicken to the pan, covering it with the sauce, and cook over low heat until the chicken is hot again.