In a shallow bowl, mix 1/2 cup flour, salt and pepper. Dip chicken breasts in flour mixture to coat both sides; shake off excess., In a large cast-iron or other heavy skillet, heat butter over medium heat. Cook chicken in batches until a thermometer reads 165°, 5-7 minutes per side. Remove chicken, reserving drippings in pan., Add broth to skillet; bring to a boil. Simmer, uncovered, until liquid is reduced to 1/3 cup, about 10 minutes. Stir in 3/4 cup cream, lemon juice and mushrooms; cook over medium-low heat 5 minutes., In a small bowl, mix the remaining flour and cream until smooth; stir into sauce. Bring to a boil; cook and stir until thickened, 1-2 minutes. Add chicken; heat through.