For the turkey breasts: In a large saucepan, heat the extra-virgin olive oil and saute the onion for a minute, or until translucent. Add the turkey to the pan, and cook for a few minutes on each side. Finish cooking the turkey by adding the wine at the very end to deglaze the pan. Season with salt and pepper, to taste.
For the sauce: In another large saucepan, heat the extra-virgin olive oil and saute the onion until translucent. Add the mixed mushrooms, and cook until soft. Season with salt and pepper, to taste, before adding the wine. Let the wine reduce, and cook for a few more minutes. Sprinkle with parsley at the very end.
Transfer the turkey to a warmed plate and scoop the mushroom sauce on top. Serve immediately.