Season the chicken breast slices with salt and pepper.
Coat each slice with flour, ensuring a thin and even layer.
Heat olive oil in a sauté pan over medium-high heat.
Add the chicken and cook for 2-3 minutes on each side or until golden brown.
Remove the chicken from the pan and set aside.
In the same pan, add the mushrooms and shallots.
Cook over medium heat until the mushrooms are browned and the shallots are tender, about 4-5 minutes.
Pour in the Marsala wine and chicken stock.
Let the mixture simmer and reduce for about 3-4 minutes while scraping any browned bits from the bottom of the pan.
Return the chicken to the pan with the reduced sauce.
Add butter and continue cooking for about 5 minutes or until the sauce slightly thickens.
The chicken should be fully cooked through.