Preheat oven to 190°C (375°F).
Boil lasagna noodles in salted water until al dente; drain and set aside individually to prevent adhesion.
In a large skillet over medium-high heat, brown the ground beef until an internal temperature of 71°C (160°F) is reached; drain excess lipids.
Add diced onion and minced garlic to the skillet; sauté for 5 minutes until translucent.
Integrate marinara sauce, salt, and pepper into the meat mixture; simmer for 10 minutes.
In a medium bowl, whisk together ricotta cheese, egg, Parmesan, and parsley until homogenized.
Layer 240ml of meat sauce at the bottom of a 9x13 inch baking vessel.
Place 4 noodles over the sauce, followed by 1/3 of the ricotta mixture, 1/3 of the meat sauce, and 1 cup of mozzarella.
Repeat the layering sequence twice more, concluding with a top layer of mozzarella cheese.
Cover with aluminum foil and bake for 25 minutes.
Remove foil and continue baking for an additional 25 minutes until the cheese is bubbly and lightly browned.
Rest the dish for 15 minutes at room temperature to allow the internal structure to set before portioning.