This is definitely not a traditional lasagna, but who cares when you can make such a good, fast approximation on a weeknight? Store-bought cheese ravioli are the ultimate hack because they let you skip all the layering and spreading. Just parboil them, tear up some fresh mozzarella, and open a jar of good marinara sauce. I saute the sauce quickly with some ground beef for a cheater Bolognese, but you can skip that step entirely for a veggie version.
In a large skillet, heat the olive oil over medium heat until shimmering. Add the beef and a generous pinch each of salt and black pepper and cook, breaking up the meat with a wooden spoon, until just cooked through, about 7 minutes. Stir in the marinara sauce and red pepper flakes and cook, stirring occasionally, until bubbling, about 5 minutes. Season the sauce with salt and black pepper.
Spread 1 cup of the sauce over the bottom of a 9-by-13-inch ceramic baking dish and arrange half the ravioli on top. Spread half the remaining sauce on the ravioli and sprinkle the arugula on top. Scatter half the mozzarella on the arugula and season with salt and black pepper. Layer the remaining ravioli, sauce, and cheese on top.
Cover the lasagna tightly with aluminum foil and bake for about 30 minutes, until the filling is bubbling and the cheese has melted. Uncover the lasagna and bake for 15 minutes more, until the top is lightly browned. Let stand for 10 minutes, then sprinkle with arugula and serve.
Do It Ahead: The unbaked lasagna can be covered and refrigerated overnight. Bring it to room temperature and then bake it as directed in step 4.