01 -
Arrange a rack in the middle of the oven and heat the oven to 400°F.
02 -
Finely chop the yellow onion. Heat olive oil in a large skillet over medium-high heat until shimmering. Add the onion, ground beef, salt, and pepper, and cook, breaking the beef into small pieces with a wooden spoon, until the beef is cooked through, 6 to 8 minutes. Remove from the heat and let cool for 5 minutes.
03 -
Spread a thin layer of marinara sauce in the bottom of a 9x13-inch baking dish. Stir the remaining sauce into the ground beef mixture.
04 -
Place 5 lasagna noodles in the baking dish, breaking them if needed to create a single layer. Dollop and spread 1 cup of the ricotta cheese over the noodles. Dollop and spread about 1 1/2 cups of the meat sauce on the ricotta, then sprinkle with 1 cup of the mozzarella.
05 -
Arrange 5 more noodles over the mozzarella, followed by 1 cup of the ricotta cheese, 1 1/2 cups of the meat sauce, and 1 cup of the mozzarella. Top with a final layer of 5 noodles and the remaining sauce, spreading the sauce thin so that it almost completely covers the noodles. (Reserve the remaining 1 cup mozzarella for the end of baking.)
06 -
Cover the dish tightly with aluminum foil. Bake for 1 hour. Check to make sure the noodles are done by poking the lasagna with a knife; the knife should slide easily through all the layers. If it doesn't, cover and cook for 15 minutes more.
07 -
Uncover the lasagna and sprinkle with the remaining 1 cup mozzarella. Bake uncovered until the mozzarella is melted and lightly browned, and the sauce is bubbling, 8 to 10 minutes more.
08 -
Let the lasagna cool on a wire rack for at least 15 minutes before serving.