Preheat oven to 375 degrees F. Bring a large pot of salted water to a boil. Cook lasagna noodles to the minimum cook time on the package. They will finish cooking in the oven. Drain, rinse with cold water, and lay flat on parchment paper to prevent sticking.

Heat olive oil in a large skillet over medium-high heat. Add onion and cook for 3 to 4 minutes until softened and translucent. Add ground beef and cook, breaking apart, until fully browned with no pink remaining. Drain excess grease. Season with salt and pepper. Add minced garlic and stir for 30 seconds until fragrant.

Add marinara sauce, Italian seasoning, and parsley to the skillet. Stir to combine and bring to a simmer. Reduce heat to low and cook uncovered for 10 to 15 minutes, stirring occasionally, to reduce excess liquid and deepen flavor.

In a medium bowl, lightly beat the egg with a fork. Add ricotta cheese and stir until smooth and fully combined. Set aside.

Spread about 1 cup of meat sauce evenly across the bottom of a 9x13-inch baking dish. Lay 3 noodles in an even layer over the sauce. Spread one third of the remaining meat sauce over the noodles. Spoon half of the ricotta mixture on top and gently spread it out. Sprinkle one third of the mozzarella and one third of the Parmesan over the top.

Repeat the same layer one more time: 3 noodles, half of the remaining sauce, the rest of the ricotta, half of the remaining mozzarella, and half of the remaining Parmesan.

For the final layer, add the last 3 noodles, the remaining meat sauce, and all of the remaining mozzarella and Parmesan. Lightly spray the underside of aluminum foil with cooking spray to prevent sticking, then cover the dish tightly.

Bake covered for 35 minutes. Uncover and bake for 10 to 15 minutes more until the top is golden, the cheese is lightly browned in spots, and the edges are bubbling actively.

Remove from oven and let the lasagna rest for 10 full minutes before slicing. This allows the layers to set for clean, neat slices.