3 whole boneless chicken breasts (about 2-1/2 pounds), sliced thin
1-1/2 tablespoons olive oil
3-1/2 tablespoons unsalted butter
2 shallots, minced
3/4 pound mushrooms, sliced thin
1/2 cup Marsala wine
1 cup chicken broth
Flour
Pat chicken dry and season with salt and pepper. Dredge chicken in flour and lay in single layer on waxed paper or plate. In a large, heavy skillet, heat oil and 1-1/2 tablespoons butter over moderately high heat until not but not smoking and brown chicken in 2 batches, transfering with tongs to a large plate as browned.
Discard all but 1 tablespoon fat from skillet and saute onion and mushrooms, stirring occasionally, until liquid mushrooms give off is evaporated. Add Marsala and cook mixture, stirring, until Marsala is almost evaporated. Add broth, chicken and any juices that have accumulated on the plate and simmer, turning chicken once, until cooked through, about 15 minutes. Transfer chicken to platter.
Simmer mushroom sauce until liquid is reduced to about 1/2 cup. Remove skillet from heat and stir in remaining 2 tablespoons butter and salt and pepper to taste, stirring until butter is just incorporated. Thicken, if desired, with 1/2 tablespoon cornstarch mixed with 1/4 cup chicken broth. Pour sauce over chicken and serve.
