Heat the vegetable stock, salt, bay leaves, peppercorns, and mustard seeds large stockpot on the stove over medium-high heat. Bring to a boil, stir occasionally until salt is dissolved. Cool to room temperature (throw in some ice to cool faster if needed).
Add herbs, garlic, onion, and half bottle of chardonnay (~11 oz.) to the pot. Place turkey breast into the pot, breast down. If turkey is not fully submerged, add enough cold water to cover. Store in refrigerator overnight or at least 12 hours.
Preheat oven to 425 degrees F. Once ready, remove turkey breast from brine and pat dry. Place breast in roasting pan (recommend one with a roasting rack), breast side up. Mix together 2 tbsp. butter, oil, and 2 tbsp. fresh sage and rub all over turkey breast including under the skin. Place a probe thermometer with a temperature alarm (highly recommended) into the thickest part of the breast. Set the thermometer alarm to 160 degrees F.
Roast turkey breast at 425 degrees F for 15 minutes and lower the oven temperature to 350 degrees F for the rest of the cooking duration. Continue roasting until probe thermometer alarm goes off when the turkey breast is 160 degrees F. A 5-6 pound turkey breast will likely take between 1 ½-2 ¼ hours. Once turkey breast has reached 160 degrees F. Allow to rest at room temperature for at least 15 minutes or until thermometer reaches 165 degrees F before carving.
While turkey is resting, add drippings to a large glass measuring cup (or degreasing cup) and skim off the fat to reserve the drippings. Add the drippings, the stock, ½ cup of chardonnay to a small pot, and 2 tbsp. minced sage (or other minced herbs of your choosing like thyme and/or rosemary) to a small pan and bring to a boil. Lower heat and simmer for about 5-7 minutes or until mixture has reduced. In a separate, small bowl mix together 2 tbsp. soften butter and all-purpose flour into a paste. Add butter-flour paste to simmering stock mixture and boil until gravy thickens. Season with salt and pepper to taste.