Place chicken between 2 sheets of heavy-duty plastic wrap; flatten to 1/4-inch thickness, using a meat mallet or rolling pin. Sprinkle both sides of chicken evenly with 1/2 teaspoon salt and 1/4 teaspoon pepper. Dredge in breadcrumbs. Set aside.
Sauté mushrooms in 1 teaspoon hot oil in a large nonstick skillet over medium-high heat 8 minutes or until edges are browned. Remove from skillet. Sprinkle with remaining 1/4 teaspoon salt and 1/4 teaspoon pepper; set aside.
Sauté garlic in 1 tablespoon hot oil over medium heat 5 to 10 minutes or until lightly browned and soft. Remove from skillet, and mash lightly with a fork or potato masher; set aside.
Prepare Lemon Mashed Potatoes, and keep warm.
Melt 1 tablespoon butter with 1/2 tablespoon oil in skillet over medium heat; add 2 chicken breasts, and cook 4 minutes on each side or until done. Remove chicken to a wire rack in a jellyroll pan. Keep chicken warm in oven at 225°. Repeat with remaining butter, oil, and chicken.
Stir chicken broth and next 3 ingredients into skillet, and cook 2 minutes, stirring to loosen particles from bottom of skillet. Stir in sautéed mushrooms and garlic.
Stir together 3 tablespoons flour and 1/4 cup water; whisk into broth mixture over medium-high heat. Cook 3 minutes, whisking constantly, or until thickened. Serve with chicken and Lemon Mashed Potatoes.