Combine tomato sauce, diced tomatoes, garlic and seasonings in a bowl and stir until mixed.
Place the chicken breasts in a crockpot or casserole dish. Pour sauce on top. Cook on high for 3 hours in a crockpot or bake at 250 degrees for 3 hours.
Add the cream cheese and spinach. Continue cooking using preferred method for another hour.
Remove chicken and bay leaf. Stir sauce until creamy. Shred chicken and add back into the sauce. Serve over spaghetti. Garnish with Parmesan cheese, if desired.