Salt and pepper both sides of chicken. Dredge each chicken breast through the flour. Set aside and heat a large pan with ¼-inch of oil.
Gently place two chicken breasts in the pan and cook until browned, about two minutes per side; transfer to a plate. Add another tablespoon of oil and repeat with the remaining chicken breasts. Add the shallots and garlic to the pan, stir and cook for about one minute or until fragrant. Stir in the mushrooms (add oil to the pan if needed) and cook until tender and slightly browned. Note: to make this an even more hands-off recipe, you can skip browning the chicken and sautéing the shallots and mushroom by adding everything into the slow cooker at once.
Transfer the chicken and mushroom mixture to the slower cooker. Add the Holland House Marsala Cooking Wine, chicken stock, lemon juice and lemon zest. Cover and set slow cooker to low for three hours.
In a small bowl, make the cornstarch slurry by whisking together cornstarch and about four tablespoons of water. After cooking time has elapsed, remove the lid and transfer the chicken to a plate. Pour in the cornstarch slurry and stir to combine. Return the chicken to the slow cooker, turn the heat to high. Cook for another 30 minutes or until sauce has thickened.
Taste and season the meal with additional salt and pepper as needed. For added flavor and garnish, top with chopped parsley and serve with noodles.