Bring a large pot of lightly salted water to a boil. Cook lasagna noodles in boiling water for 8 to 10 minutes. Drain noodles, and rinse with cold water. In a mixing bowl, combine ricotta cheese with egg, 2 tablespoons parsley, and 1/2 teaspoon salt. Preheat the oven to 375 degrees F (190 degrees C). To assemble, spread 1 1/2 cups of meat sauce in the bottom of a 9x13-inch baking dish. Arrange 6 noodles lengthwise over meat sauce. Spread with 1/2 of the ricotta cheese mixture. Top with 1/3 of the mozzarella cheese slices. Spoon 1 1/2 cups meat sauce over mozzarella, and sprinkle with 1/4 cup Parmesan cheese. Repeat layers, and top with remaining mozzarella and Parmesan cheese. Cover with foil: to prevent sticking, either spray foil with cooking spray or make sure the foil does not touch the cheese. Bake in the preheated oven for 25 minutes. Remove the foil and bake for an additional 25 minutes. Rest lasagna for 15 minutes before serving.
Heat olive oil in a skillet over medium heat; cook and stir onions and Italian sausage until the sausage is browned, about 10 minutes. Transfer the sausage and onions to a slow cooker. In the same skillet, cook and stir the ground beef, Italian seasoning, 1 teaspoon of garlic powder, and marjoram, breaking the meat up as it cooks, until the meat is browned, about 10 minutes. Transfer the ground beef into the slow cooker. Stir in the tomato sauce, tomato paste, diced tomatoes, stewed tomatoes, thyme, basil, oregano, and 2 teaspoons of garlic powder. Set the cooker on Low, and cook the sauce for 8 hours. About 15 minutes before serving, stir in the sugar. Serve hot.