I use frozen kale in this recipe because the frozen kale adds a little more liquid to the Dutch oven while it cooks. That gives the pasta a little more liquid to cook it, which helps overall. If you can't find frozen kale, or just happen to have fresh kale handy, you can use that instead. If you use fresh kale, add an extra 1/4 cup of beef broth (or just plain water) and a 1/8 teaspoon salt when you add the beef broth. And, if you don't like kale or can't find frozen kale in your grocery store, you can use frozen spinach instead.