Trim, peel and chop the eggplant. Put it in a colander and toss with the two teaspoons salt. Let stand for about 30 minutes. Rinse with cool water, drain and pat dry. Heat the olive oil in a skillet over medium heat. Cook the eggplant, stirring often, until it is soft and tender. Remove from heat and set aside. In another skillet, brown the ground beef. Add the onions, bell peppers and garlic and cook for about five minutes, or until the vegetables are soft. Add the eggplant. Season with salt, cayenne and black pepper. In a mixing bowl, combine the egg, ricotta, shredded mozzarella and Parmesan cheese. Fold this mixture into the beef mixture and mix well. Remove from heat. Cook the manicotti, in two batches, in boiling, salted water until tender, about five to seven minutes. Drain the manicotti and place in a bowl of cool water until ready to use.

Preheat the oven to 350°F. Lightly butter a 9x13-inch or larger baking dish. Drain the manicotti and pat dry with a paper towel. Fill each manicotti with the eggplant and beef mixture. Arrange in the baking dish and pour the tomato sauce over the top and then top with the slices of mozzarella cheese. Cover with foil and bake for 30 minutes. Remove the foil and bake for 15 minutes more. Serve hot, with more pasta sauce if desired.