Stir oil and garlic together in the bowl of a slow cooker.
2
Mix flour, salt, and pepper together in a large resealable plastic bag. Add chicken to the bag, seal, and shake to coat chicken with flour mixture. Pour coated chicken into the slow cooker, then add mushrooms.
3
Heat Marsala wine in a small frying pan over medium-high heat until reduced, 2 to 3 minutes; pour into the slow cooker over chicken and mushrooms.
