This simple 16th century ragu of slow-cooked onions and beef is deeply flavored and comforting, but we had to tweak it slightly to make it work in a modern context. Originally, the recipe made two meals or courses: a rich, savory sauce for pasta and a dish of cooked beef. In our version we shred the meat and add it back to the sauce for one substantial meal. To eliminate the need for intermittent stirring and monitoring during cooking, we move the action from the stovetop to the oven. We add tomato paste for color and to boost the flavor, and we make sure to keep the mixture wet enough to extract maximum flavor from the onions. To encourage the formation of a sauce that clings to the pasta, we vigorously stir them together, so that the starch from the pasta adds body to the sauce.