Clean mushrooms with a damp cloth of any dirt or debris and trim of the ends. Slice
Peel and dice onion
Season chicken breasts with salt and pepper
In a heavy pan, heat walnut oil and sear chicken breasts on high heat
Put pan in oven and bake for 22-25 minutes
Carefully remove pan from oven and set chicken breasts aside on a plate
If necessary, add a little more walnut oil to the pan
Add onion and brown quickly on high heat
Add mushrooms to pan and continue to cook for another five minutes on high heat
Dissolve the mustard into the cider and add, lowering the heat to low/medium heat and simmer another five minutes, using the cider to deglaze the pan.
Stem the fresh thyme and add to pan, stirring constantly.
Whisk the corn starch into the cold heavy cream until fully dissolved. Slowly add to the pan and mix thoroughly. Stir frequently until the sauce starts to thicken. Add the chicken breasts to coat
Plate the chicken on a large platter and pour sauce over the chicken. Serve.