Place the beef in a pot and cook over medium heat until cooked through. Drain the excess fat. Place the beef in a medium bowl and cool to room temperature. Cook the pasta shells as per package directions, drain well, cool in cold water and drain well again.
When the beef has cooled, mix in the ricotta, spinach, eggs, pesto, salt and pepper. Preheat the oven to 350 degrees F. Combine the marinara sauce and stock into a 9" x 13" casserole. Stuff the beef mixture into the shells and set in the casserole. Sprinkle the shells with the Parmesan cheese. Cover and bake 35 minutes. Uncover and bake 10 to 15 minutes more, or until the tops of the shells are golden and the sauce is bubbling.
Marinara sauce is available in the aisle where pasta sauce and tomato products are found.
Options: Instead of beef, use another ground meat in this recipe, such as pork or veal. Instead of marinara sauce, bake the shells in any other tomato-based pasta sauce that appeals, such as tomato basil or roasted red pepper. Instead of Parmesan, top the shells with another type of grated cheese, such as mozzarella or provolone, increasing the amount used as desired.