Preheat oven to 200 degrees.

Heat three tablespoons of olive oil in a large skillet to medium-high heat. Salt and pepper chicken, then lightly dredge in the flour, shaking off any excess. Place in skillet and saute until just golden brown, two to three minutes. Turn and cook on the other side, 2 minutes. You may need to work in batches.

Reduce heat to medium, remove chicken from the pan, and place in oven, covered, to keep warm. Add two more tablespoons of olive oil to the pan and add mushrooms. Sauté, stirring occasionally (about 5 to 6 minutes) until golden brown. (If mushrooms become too dry, add a tablespoon or so of water.) Add garlic and onion and cook for a minute or two longer.

Remove pan from heat and add the Marsala, then return the pan to the heat and stir until the sauce comes to a boil, scraping the bottom of the pan.

Reduce to a simmer and simmer until the sauce is slightly thickened, two to three minutes. Turn off the heat and add the tablespoon of butter, slowly stirring it into the sauce. Add the chicken, turning to coat the chicken in the sauce.

Place chicken and mushrooms on a platter, pour remaining sauce over the chicken, and sprinkle with parsley.