This simple lasagna is also one of the most spectacular—start with the best ingredients, and it’s simpler than you might think.
In a large, heavy skillet, heat a drizzle of oil and sauté the onion for 4-5 minutes, until soft. Add the garlic and oregano and cook for another minute, then add the sirloin 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.
Preheat the oven to 350˚F. Spread 1 cup of the sauce in the bottom of a 9x13-inch pan. Top with a layer of lasagna sheets, another cup of sauce, some of the ricotta and some of the mozzarella. Repeat with pasta sheets (break or cut them to fit if necessary), sauce and cheeses. Put all the ricotta in between the first two layers, then top the last pasta sheet with sauce and mozzarella, and then a scattering of Parmesan.
Cover with foil and bake for 45 minutes, then uncover and bake for an additional 30 minutes, until bubbly and golden.