Preheat the oven to 350 degrees F. In a large pot of boiling, salted water, cook the manicotti tubes according to package directions. Drain, rinse in cold water, and pat dry. In a nonstick skillet set over moderate heat, warm the oil until hot. Add the onion and cook it, stirring occasionally, for 5 minutes. Add the meat, lemon pepper, oregano, salt, and pepper, and cook, stirring, until the meat is no longer pink. Transfer the meat to a bowl, add the parsley, tomatoes, ricotta, and half the Parmesan, and fill the pasta tubes with the meat mixture. Spoon 1 cup sauce in bottom of shallow baking dish, top with the filled pasta tubes, and spoon the remaining sauce over them. Sprinkle with Parmesan and bake for 25 to 30 minutes, or until sauce is bubbling.