Cut chicken breasts in half lengthwise to create 2 thin pieces from each breast. Pound the chicken pieces to about 1/4 inch thick using a meat mallet or rolling pin.

Heat olive oil in a large 12-inch deep skillet over medium-high heat until shimmering. On a plate, whisk together flour, salt, and pepper.

Coat each piece of chicken with the flour mixture, shaking off excess. Cook 3 pieces at a time for 3-4 minutes on each side until internal temperature reaches 165°F and golden brown. Remove and set aside.

In the same skillet, melt butter over medium heat. Add mushrooms and onions, cooking for 4-5 minutes until softened.

Add 1 tablespoon flour and garlic. Stir and cook for 1 minute, scraping the bottom of the pan to release browned bits.

Pour in Marsala wine and simmer for 1-2 minutes. Whisk in chicken broth and heavy cream.

Bring to a gentle boil, then reduce heat and simmer for 8-10 minutes, stirring occasionally, until sauce thickens enough to coat the back of a spoon.

Return chicken to skillet and simmer for 2-3 minutes until heated through. Taste and adjust seasoning. Serve immediately.