Bring a large pot of salted water to a boil and cook the lasagna noodles until al dente. Drain and set aside.

Heat the olive oil in a large skillet over medium heat. Add the diced onion and minced garlic, then cook until the onion softens.

Add the ground beef to the skillet and cook until browned, breaking it up with a spoon. Drain excess fat if needed.

Stir in the tomato sauce, oregano, salt, and pepper. Simmer the meat sauce for 10 minutes.

In a bowl, combine the ricotta cheese with half of the grated Parmesan.

Spread a thin layer of meat sauce in the bottom of a baking dish. Add a layer of noodles, then half the ricotta mixture, followed by one-third of the mozzarella. Repeat the layers once more.

Finish with the remaining noodles, meat sauce, mozzarella, and Parmesan.

Bake at 375°F for 25 minutes until the cheese is bubbly and lightly browned. Let the lasagna rest for 10 minutes before slicing.