01 - Preheat the oven to 375°F. Pat the turkey breast completely dry with paper towels to ensure proper skin crisping.
02 - In a small bowl, combine softened butter, olive oil, chopped rosemary, thyme, sage, garlic, lemon zest, salt, and pepper. Mix thoroughly to form a cohesive herb paste.
03 - Gently loosen the skin from the turkey breast using your fingers. Rub half the herb paste directly under the skin, then spread the remaining paste evenly over the exterior surface.
04 - Arrange the quartered onion, carrot chunks, and celery stalks in the bottom of a roasting pan. Pour 1 cup of chicken broth over the vegetables.
05 - Place the turkey breast skin-side up on the vegetable bed. Roast uncovered for 1½ hours, basting with pan juices every 30 minutes, until the internal temperature reaches 165°F in the thickest portion.
06 - Transfer the roasted turkey breast to a serving platter. Tent loosely with aluminum foil and allow to rest for 15 minutes before slicing to retain juices.
07 - Strain the pan drippings, discarding the cooked vegetables. In a saucepan over medium heat, melt 2 tablespoons of butter.
08 - Whisk the cornstarch into the melted butter to form a roux. Gradually incorporate 2 cups of broth and 2 tablespoons of reserved pan drippings while whisking continuously.
09 - Simmer the gravy for 5-7 minutes, whisking constantly until thickened. Season with salt and pepper to taste. Slice the rested turkey and serve accompanied by warm herb gravy.