01 - Preheat oven to 375°F.
02 - Heat olive oil in a large skillet over medium heat. Add onion and cook 3 to 4 minutes until softened. Stir in garlic and cook 1 minute. Add ground beef, salt, pepper, oregano, basil, and red pepper flakes; cook, breaking up meat, until browned. Drain excess fat if necessary. Stir in tomato paste and cook 1 minute. Add crushed tomatoes, tomato sauce, and sugar. Simmer uncovered for 20 to 25 minutes, stirring occasionally until thickened. Remove from heat.
03 - In a medium bowl, combine ricotta, egg, Parmesan, parsley, salt, and pepper. Stir until smooth.
04 - Cook lasagna noodles according to package directions if using regular noodles. Drain and set aside. Skip this step if using no-boil noodles.
05 - Spread 1 cup of meat sauce on the bottom of a 9x13-inch baking dish. Layer 4 noodles over the sauce. Spread one-third of the ricotta mixture over noodles, sprinkle with 1 cup mozzarella cheese, then top with 1 1/2 cups meat sauce. Repeat layering twice, finishing with noodles and remaining meat sauce. Top with remaining mozzarella and Parmesan cheeses.
06 - Cover the dish with foil, spraying the underside with oil to prevent sticking, and bake for 25 minutes.
07 - Remove foil and bake an additional 20 to 25 minutes until cheese is golden and bubbly.
08 - Let rest for 15 minutes before slicing and serving.