Place turkey breast on flat surface. Cover with saran wrap or plastic bag, and use a mallet to pound to even thickness, about 1-/2 inch thick in all places.
In a small bowl, Mix the water and cornstarch together until the cornstarch is fully dissolved. Set aside.
In a large skillet, melt butter on medium- low heat. Meanwhile, add flour and garlic powder to a bowl. Quickly dredge the turkey breast in the flour mixture on each side, then place in the hot skillet.
Cook on medium for about 5 minutes, then flip, and cook for another 5 minutes.
Remove turkey to a plate and reduce skillet heat to low.
Add mushrooms and cook for 2-3 minutes, until the mushrooms are just beginning to caramelize.
Add lemon juice, wine, and cornstarch mixture. Stir and heat until simmering, about 1-2 minutes.
Add capers and parsley, and spoon sauce over turkey breasts. Serve with fresh lemon wedges (optional).