This recipe has been updated and may differ from what was originally published or broadcast.
Heat the olive oil in a medium pot, then add the onions, garlic and red pepper flakes and season with salt. Cook, stirring occasionally, until golden. Add the tomatoes and bay leaf, bring to a simmer and simmer for 20 to 25 minutes. Season to taste, then discard the bay leaf.
Preheat the oven to 350ºF.
Bring a large pot of water to a rolling boil. Stir in a generous handful of salt and add the lasagna sheets. Cook, stirring so they don’t stick together, until still very firm to the touch, about 4 minutes, then drain and rinse under cold water. Separate the sheets carefully so they don’t stick together.
Spoon a thin layer of the sauce in the bottom of an 8-inch square baking pan. Arrange a layer of the pasta sheets over the sauce. Sprinkle some of the mozzarella, Parmesan and basil leaves over the pasta, then add another layer of sauce. Repeat the layering process 2 more times, ending with pasta and cheese.
Cover the baking pan tightly with aluminum foil and place in the center of the oven. Bake for 45 minutes, then raise the temperature of the oven to 450ºF and remove the aluminum foil. Bake until the top browns slightly, an additional 10 to 15 minutes. Let cool slightly, about 10 minutes, then cut into squares. Drizzle with the balsamic and top with chopped parsley.