It’s OK, you CAN carefully defrost this from frozen, but it takes a little more work to reheat, which you might not want to do on Christmas Day. You can either pop it into the slow cooker for about 1 hour on HIGH or 2 hours on LOW.  You can do it in the oven in a lidded casserole dish, on 180C for 20 minutes or so. On the hob, gently heat in a lidded saucepan for about 10 - 15 minutes (it might need a little water). You can also do it in the microwave for approximately 10 minutes. Stir occasionally for each method and make sure the cabbage is heated all the way through before serving.