Peel the garlic cloves, and crush them with the heel of your hand on a cutting board. A longer cooking time is needed to soften the larger pieces and mellow the flavor. Grate the lemon rind for the potatoes before you squeeze the fresh juice for the chicken.
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.