Place the chicken in the slow cooker and then sprinkle salt, pepper, and garlic on top. Add mushrooms and sun-dried tomatoes, then pour the marsala wine and chicken broth on top. Cook for 5 to 6 hours on low or 2 to 3 hours on high.
Remove the chicken from the slow cooker and transfer to a plate.
Using a fork, mix together softened butter and flour, add to the slow cooker and whisk to combine with liquid. Transfer chicken back to the slow cooker and cook on high 15 minutes or until gravy thickened.