Heat the olive oil in a large skillet over medium-high heat until shimmering, then add the ground beef and cook for 5-7 minutes, breaking it apart with a wooden spoon until no pink remains.

Add the diced onion and sauté for 3-4 minutes until softened, then add the minced garlic and cook for 1 minute until fragrant.

Stir in the tomato paste and cook for 2 minutes, then add the crushed tomatoes, Italian seasoning, salt, black pepper, red pepper flakes, and sugar; bring the sauce to a simmer.

Reduce heat to low and simmer the sauce uncovered for 15-20 minutes, stirring occasionally until slightly thickened.

Meanwhile, in a medium mixing bowl, combine ricotta cheese, egg, 1.5 cups of mozzarella, Parmesan cheese, salt, pepper, and Italian seasoning; stir until smooth and well combined.

Bring a large pot of salted water to a boil, then add the lasagna noodles and cook until al dente according to package directions; drain and lay the noodles flat on a clean towel.

Preheat oven to 375°F (190°C). Spread a thin layer of meat sauce in the baking dish, then layer noodles, one-third of the ricotta mixture, and one-third of the meat sauce; repeat two more times.

Top the final layer of noodles with the remaining meat sauce and sprinkle with 1 cup of shredded mozzarella cheese.

Cover the baking dish tightly with aluminum foil and bake for 25 minutes in the preheated oven.

Remove the foil and bake uncovered for an additional 20-25 minutes until the cheese is melted, golden brown, and the sauce is bubbling around the edges.

Remove from the oven and let the lasagna rest for 15 minutes before slicing and serving to allow the layers to set.