Combine frozen vegetable medley in the bottom of a 5-quart slow cooker. Season both sides of the chicken breasts with salt, pepper, and Italian seasoning. Place chicken on top of vegetables.
In a large bowl, stir together pasta sauce, tomato paste, garlic and sherry. Pour 2/3 of the sauce over chicken and vegetables. Top the chicken with polenta slices and cover with remaining sauce. Cover and cook on the LOW setting for 4 to 5 hours.