Preheat the oven to 350°F.
In a large, heavy skillet, heat the oil over medium-high heat and sauté the onion until soft.
Add the garlic and cook for another minute, then add the beef and cook, breaking the meat up with a spoon, until it's no longer pink.
Season with salt and pepper, add the tomato sauce and simmer for a few minutes.
Spread about 1 cup of the sauce in the bottom of a 9x13-inch or lasagna pan.
Top with a layer of uncooked lasagna noodles, about a third of the sauce, half the ricotta cheese (dropped in spoonfuls over the sauce) and a handful of mozzarella.
Repeat with lasagna noodles, sauce, ricotta and cheeses.
Top with a third layer of noodles, sauce, and the remaining mozzarella along with the Parmesan.
Cover with foil and bake for 45 minutes, then uncover and bake for an additional 30 minutes, until bubbly and golden.
Let sit for at least 15 minutes before slicing.