Preheat your oven to 350°F. Lightly grease a square 8-inch baking dish.
Defrost the spinach in the microwave. Drain thoroughly by transferring the thawed spinach to a colander and pressing on it repeatedly with the back of a large spoon to extract as much liquid as possible. Set aside.
Heat the olive oil in a large skillet over medium-high heat for about 3 minutes. Add the beef, onion, and ½ teaspoon of kosher salt. Cook, using a wooden spoon to stir and break up the ground beef into small chunks, until the onion is soft and the meat is browned, 5-7 minutes.
Drain the beef mixture, then return it to the skillet. Add the garlic and cook, stirring, for 30 seconds, then mix in the marinara sauce. Turn the heat off.
Place the cream cheese in a medium microwave-safe bowl and heat for 40 seconds to soften. With a fork, stir it until smooth and creamy. Add the egg, the black pepper, and the remaining ½ teaspoon of kosher salt and keep stirring the mixture until very smooth. With a spatula, add the drained spinach and mix to blend everything together. Mix well.
Spread the spinach mixture evenly on the bottom of the prepared baking dish. Top with the meat mixture. Sprinkle the mozzarella on top. Bake the lasagna until the cheese is bubbly and lightly browned, about 30 minutes. Let stand for ten minutes before cutting and serving.
Video
Notes
It's important to remove as much liquid as possible from the spinach to prevent watery lasagna. You should drain it thoroughly by placing it in a colander and pressing on it repeatedly with the back of a large spoon.You can keep the leftovers in the fridge, in a sealed container, for 3-4 days. Reheat them gently in the microwave at 50% power. You can also cut the lasagna into individual portions and freeze them in freezer bags for up to 3 months.