How to Make It
Preheat the oven to 425°. In a large pot of salted boiling water cook the lasagna noodles until they are al dente, about 6 minutes. Drain the noodles, rinse them under cold running water and set aside.
In a large frying pan, heat the extra-virgin olive oil over medium high heat. Add the onion and cook for 5 minutes, until it is soft and beginning to brown. Add the garlic and cook for 30 seconds. Add the chopped mushrooms and cook, stirring occasionally, for 10 minutes, until the mushrooms are soft and browned. Add the spinach and cook until it wilts, about 1 minute. Add the marinara sauce, balsamic vinegar and season generously with sea salt and freshly ground pepper.
In a medium bowl, whisk together the ricotta, egg, sea salt, freshly ground pepper and nutmeg.
In an 8-inch-square baking dish, spread 1/2 cup of the ricotta mixture and top it with 4 lasagna noodles, overlapping them slightly. Spread half of the spinach and mushroom mixture on top of the noodles and cover with half of the ricotta. Sprinkle with 1 cup of the Parmigiano-Reggiano cheese. Arrange another layer of 4 lasagna noodles, then top them with the remaining spinach and mushrooms and ricotta. Sprinkle with the rest of the Parmigiano-Reggiano and dot the lasagna with the fresh mozzarella.
Cover the baking dish with nonstick aluminum foil and bake in the preheated oven for 35 minutes. Remove the foil, turn the oven to broil and broil the lasagna for 5 to 10 minutes, until the top is brown and bubbly.
Remove the lasagna from the oven and let it rest for at least 5 minutes before serving.