Cook the pasta shells as per package directions. While this occurs, combine the next 6 ingredients in a bowl. Preheat the oven to 350 degrees F. When the shells are cooked, drain well, cool in cold water, and drain well again. Pour the pasta sauce into a large, wide casserole; stir in the stock. Stuff the ricotta mixture into the pasta shells and set on top of the pasta sauce. Sprinkle shells with Parmesan cheese. Cover and bake 30 minutes. Then uncover and bake 10-15 minutes more, or until the tops are golden and the sauce is bubbling.