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, or until it is soft and beginning to brown. Add the garlic and cook for 30 seconds. Add the ground beef and cook, stirring occasionally, for 10 minutes, until the beef starts to brown. 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 with 4 lasagna noodles, overlapping them a little. Spread half of the marinara sauce on top of the noodles, cover with half of the ricotta and sprinkle with half of the Parmigiano-Reggiano cheese. Arrange 4 more lasagna noodles on top, add the remaining marinara sauce and ricotta. Sprinkle the rest of the Parmigiano-Reggiano cheese over top and dot with the fresh mozzarella.
Cover the baking dish with nonstick aluminum foil and bake it 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.