1In a large, heavy skillet, heat the oil over medium-high heat and saute the onion until soft. Add the garlic and oregano and cook for another minute, then add the beef and cook, breaking the meat up with a spoon, until it's browned and no longer pink. Season with salt and pepper. Add the tomato sauce and simmer for a few minutes.
2Preheat the oven to 350F. Spread 1 cup of the sauce in the bottom of a 9x13-inch or lasagna pan. Top with a layer of lasagna sheets, another cup of sauce, some of the ricotta cheese and some of the mozzarella. Repeat with pasta sheets (I cut mine to fit), sauce and cheeses. I put all the ricotta in between the first two layers, then top the last pasta sheet with sauce and mozzarella, and then the Parmesan.
3Cover with foil. (Emily recommends putting your lasagna on a baking sheet, but I like to live dangerously.) Bake for 45 minutes, then uncover and bake for an additional 30 minutes, until bubbly and golden.