You have to show the cabbage who’s boss. That’s another way of saying, we don’t really know. Did you get a particularly tough cabbage? Using traditional methods, you discard the outer leaves, which are tougher and also quite likely to be torn or damaged and not ideal for stuffing anyway. You blanch the leaves for 2 minutes in boiling water and then plunge them into cold water to stop the cooking.
An alternate method is to core the head, rinse it, then freeze it. After it thaws, proceed as normal with your recipe. The leaves will have been mercilessly beaten (frozen) into submission