In a gallon size ziploc bag, combine buttermilk and salt. Stir to dissolve salt.
Add turkey breast to bag and turn to coat in buttermilk.
Refrigerate turkey breast in brine overnight, 12-24 hours.
1 hour before you plan to bake, remove turkey breast from the brine, lightly shaking off excess brine. Place on a plate and bring it to room temperature.
Preheat oven to 350° with a shelf in the middle and one near the top. Bake turkey breast on the middle shelf for the first half of baking then move to the top shelf until internal temperature of 160° in the thickest part of the breast. Total roasting time will be around 20 minutes per pound for boneless turkey breast or 25 minutes per pound for bone-in turkey breast. If skin is browning too much towards the end of baking, tent with foil and transfer back to middle shelf.
Transfer turkey breast to a cutting board and tent loosely with foil. Allow to rest for about 15-20 minutes before carving. Reserve pan drippings, if desired, for making gravy.
While turkey is resting, make gravy: Add 1½ cup chicken stock to the roasting pan with the vegetables. Gently scrape any browned bits stuck to the pan.
Place a fine mesh sieve over a bowl or large spouted measuring cup. Pour contents of the roasting pan into the sieve and collect the drippings below.
Melt butter in a medium saucepan over medium heat. Add mushrooms to saucepan and cook, stirring occasionally, for about 5-7 minutes or until mushrooms are softened and starting to brown.
Add salt, pepper, and flour to mushrooms, and cook, stirring continuously, for 1 minute.
Gradually pour chicken stock to the saucepan while stirring. Bring to a boil over high heat then reduce heat as needed to maintain a simmer. Cook, stirring frequently, until thickened to coat the back of a spoon, about 5-10 minutes. Remove from heat. Add additional chicken stock as needed if gravy over-thickens.
With a sharp chefs knife or a serrated knife, cut along the breast bone to remove it. Carve turkey against the grain and serve with gravy.