This hefty casserole is layered with three cheeses, a meaty tomato sauce and -- wait for it -- kale. With that preface, let me add that this recipe can easily be made vegetarian by simply omitting the meat from the tomato sauce. Add the beef (if using) and cook until lightly browned, 5 to 7 minutes, stirring as needed. Add the onion and cook until softened, about 3 minutes, stirring often. Place a layer of lasagna sheets over the sauce, breaking them to fit to size as necessary.