Curly kale, Brassica olercea, has frilly thick leaves that can be used in salads, or boiled, sauteed. It can be placed in soups or used to stuff pork. If you prefer the flavor to be mild you can blanch it first before proceeding with your desired recipe.
If you don't have curly kale you can substitute:
Another type of kale such as Tuscan Kale (Cavalo Nero) OR>Collard greens - easy to find but take longer to cook OR>Green chard - easy to find and cooking time is about the same OR>Mustard greens- a more peppery flavor OR>Spinach - cooks very fast, softer, mild flavor.