Place 1 chicken breast inside a gallon zip-top bag and pound with the flat side of a meat mallet or rolling pin to an even ½-inch thickness. Set aside and repeat with remaining chicken breasts. Season both sides of the chicken breasts with salt and pepper, then arrange chicken breasts in a single layer in a 6-quart or larger slow cooker, scattering garlic and mushrooms over the chicken, then pouring in the Marsala.

Cover and cook on the LOW setting for 2 to 3 hours, or the HIGH setting for 1 to 2 hours, until the chicken registers 165°F.

Using tongs, transfer the chicken to a plate and cover with aluminum foil to keep warm. Pour the sauce into a large frying pan, add the butter, and bring to a simmer over medium heat. Whisk the water and cornstarch together in a small bowl until dissolved, then pour into the simmering sauce. Cook until sauce thickens, 5 to 7 minutes.

Whisk in the heavy cream and lemon juice. Return the chicken to the sauce, turning to coat. Serve over cooked pasta or salad greens, garnishing with fresh parsley and lemon wedges if desired.