Preheat oven to 400 degrees, with rack in center. Place bread and sage leaves in the bowl of a food processor; pulse to form fine crumbs. Transfer to a medium bowl, and add ground turkey.
Place onion and celery in bowl of food processor; pulse until finely chopped. Add to turkey mixture, using hands to combine. Add thyme, whole egg, 1 tablespoon tomato paste, mustard, 1 teaspoon Worcestershire, and salt; season with pepper. Combine well. Place in a 9-by- 5-by-2 1/2-inch nonstick loaf pan.
In a small bowl, combine egg white with remaining tablespoon tomato paste and 1/4 teaspoon Worcestershire, whisking with a fork until smooth. Spoon mixture over meatloaf; spread to coat evenly.
Transfer pan to oven; place a baking sheet on lower rack to catch drippings. Cook until a meat thermometer inserted in center of meatloaf registers 180 degrees, about 1 1/4 hours. Remove from oven; let rest 15 minutes covered with foil.
To serve, cut meatloaf into 12 slices, and divide evenly among six serving plates. Serve immediately with mashed potatoes and hot gravy.