Flatten chicken to 1/2-in. thickness. In a large shallow dish, combine 1/4 cup flour, salt and pepper. Add chicken, 1 piece at a time; turn to coat. , In a large nonstick skillet over medium heat, cook chicken in butter for 5-6 minutes on each side or until no longer pink. Remove and keep warm., Add 1/3 cup broth to the pan, stirring to loosen browned bits. Bring to a boil. Add mushrooms; cook and stir for 3-5 minutes or until tender. , Combine the remaining flour and broth until smooth; stir into the mushroom mixture. Bring to a boil; cook and stir until thickened, about 2 minutes Stir in lemon juice. Serve with chicken.