Preheat the oven to 180°C (th.6).
Peel and chop the onion and garlic. Clean the mushrooms and dice them. Wash and dry the spinach leaves.
Heat the oil in a large pan over high heat. Sauté the lardons, onion, and garlic for a few minutes until the onion is soft. Add the ground beef and cook until all the liquid evaporates.
Stir in the mushrooms and cook for 5 minutes. Deglaze with red wine, then add the tomato purée, tomato paste, and dried oregano. Bring to a boil, then reduce the heat. Simmer uncovered until thickened. Season and set aside.
In a bowl, whisk together the ricotta and cream, add the grated mozzarella, and mix well. Lightly butter a large baking dish. Place lasagna sheets at the bottom, cutting to fit. Spread one-third of the meat sauce on top. Add one-third of the ricotta mixture, followed by half of the spinach.
Repeat the layers, ending with a layer of lasagna sheets topped with the remaining ricotta mixture. Sprinkle with Parmesan and nutmeg.
Bake for 40 to 45 minutes. Sprinkle with fresh oregano leaves just before serving.