Pound chicken breasts to 1/4 thick. Sear in oil ~5?6 min per side until cooked through; set aside.
In same pan, saut? sliced mushrooms and onion in butter until tender. Deglaze with sherry; season with salt and pepper. Stir cornstarch into milk and add to pan. Boil 1?2 min until sauce thickens.
Return chicken to pan and heat through in the creamy mushroom sauce..