Fold plastic wrap around chicken and pound out to approximately 1/2 inch thick; set aside.
Mix flour, salt, and pepper in a bowl. Coat chicken with seasoned flour and set aside on a clean platter.
Heat oil in a skillet over medium. Add chicken and brown approximately 4-5 minutes per side.
While chicken cooks, halve onions and mince garlic; set both aside. Remove chicken and place on a clean plate.
To the same skillet, add sliced mushrooms and cook for 4-5 minutes. Add onions and garlic and sauté for 3-4 minutes more, allowing onions to start to brown.
Add marsala. Stir and cook, scraping the bits of cooked food from the bottom of the pan. Add broth and boil for 1-2 minutes; sauce will begin to thicken.
Add butter and stir until melted. Add chicken and tomatoes and allow chicken to reheat.
Serve with Italian bread on the side.