Step 1Prick the sausages at 1/2-inch intervals. Heat the oil in a 4-quart pot over medium-low heat. Add the sausages and slowly brown them on all sides for about 10 minutes; remove to a plate.
Step 2Cut the pancetta into large dice. Add the pancetta, onion, garlic, pepper flakes, sage and rosemary and cook until tender, 15 minutes. Add the tomato paste, wine and beef broth and cook for 5 minutes. Add the chopped tomatoes and cook for 5 minutes more. Return the sausages and any remaining juices to the pot and simmer for 10 minutes.
Step 3Remove the pot from the heat and transfer the sausages to a cutting board. Slice them lengthwise, then cut crosswise into half-moons. Return the sausages to the pot, along with any juices left on the cutting board. Add the parsley and set aside.
Step 4Heat the oven to 350 degrees. Layer one-half cup of polenta, one-fourth of the ragu and one-third of the mozzarella in a 9-inch square baking dish. Sprinkle with a third of the Parmigiano. Repeat for 2 more layers, then top with the remaining ragu. Bake for 1 hour, 15 minutes. Let cool slightly, then cut into 6 servings.