Add the chicken breast to the bottom of the slow cooker along with the mushrooms.
Season the chicken breast and mushrooms with salt, pepper, garlic, and shallot and stir to evenly coat.
Pour in the chicken broth and marsala wine into the slow cooker over the chicken and mushrooms.
Place the lid on the slow cooker and cook on low for 4-5 hours. The chicken should read an internal temperature of 165 F.
Spoon out about 1/4 cup of liquid and whisk with the arrowroot starch. Pour back into the slow cooker for cook for another 15 minutes to allow the liquid to thicken. Add more broth if a thinner sauce is desired.
Remove the lid and top with the cooked chicken marsala with fresh parsley. Serve with a pasta of your choice like zucchini noodles or spaghetti squash.
Notes
For the marsala wine, cooking does remove alcohol. However, if you choose to forgo it, simply swap 2 tbsp of sherry or balsamic vinegar and add an extra 1/4 cup of chicken broth.All nutritional information are estimations and will vary. Estimations do not include optional ingredients.