1. Preheat oven to 450°.
2. To prepare turkey, combine first 8 ingredients in a large bowl; add turkey, turning to coat. Marinate at room temperature 20 minutes. Remove turkey from marinade; discard marinade. Place a rack inside a roasting pan; coat rack lightly with cooking spray. Arrange turkey breasts on rack. Bake at 450° for 25 minutes or until a thermometer inserted in thickest part registers 155°. Remove from oven. Let stand 10 minutes; cut turkey diagonally across the grain into 16 slices.
3. To prepare sauce, heat a medium nonstick skillet over medium-high heat. Add 2 teaspoons oil; swirl to coat. Add onion and garlic to pan; sauté 4 minutes, stirring occasionally. Combine stock and flour in a bowl, stirring with a whisk. Add stock mixture to onion mixture, stirring with a whisk. Bring to a boil; cook 2 minutes or until slightly thick, stirring constantly. Remove from heat; stir in 1/4 teaspoon salt and 1/4 teaspoon pepper. Serve sauce with turkey.