Dried cherries and sage tucked under the skin of the turkey breast give each serving a hint of sweetness mixed with a fabulous herb flavor.
Preheat oven to 325 degrees F. Place dried cherries, 1 tablespoon olive oil and sage in a blender container or food processor bowl. Cover and blend or process just until cherries are finely chopped.
Slip your fingers between the skin and meat of the turkey breast to loosen the turkey skin. Lift the skin and, using a spatula, carefully spread the cherry mixture directly over the turkey breast meat. Place the turkey breast on a rack in a foil-lined shallow roasting pan. Place the lemon and onion quarters underneath the turkey breast. Insert a meat thermometer into the breast without the thermometer touching the bone. Brush with remaining 1 tablespoon oil.
Roast, uncovered, in preheated oven for 1 1/2 to 2 1/4 hours or until thermometer registers 170 degrees F, covering with foil the last 45 minutes of roasting to prevent overbrowning. Cover and let stand for 15 minutes before carving.
Meanwhile, place mushrooms in a small bowl. Pour boiling water over mushrooms to cover. Let stand 10 minutes. Drain mushrooms. Strain the mushroom-soaking liquid through a fine-mesh sieve lined with 100%-cotton cheesecloth, paper towels or a coffee filter and reserve. Rinse and drain mushrooms a second time.
Pour pan drippings from turkey into a large measuring cup. Skim and reserve fat from drippings. Pour 1/4 cup of the fat (if necessary, add butter) into a medium saucepan. Stir in flour. Add enough mushroom liquid and broth to remaining drippings in the measuring cup to equal 2 cups. Add broth mixture all at once to flour mixture. Cook and stir over medium heat until thickened and bubbly. Stir in mushrooms and sherry. Cook and stir 1 minute more. Season to taste with salt and freshly ground pepper.