Preheat your oven to 350 degrees 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, about 3 minutes. Add the beef, onion, and ½ teaspoon of kosher salt. Cook, using a wooden spoon to stir and to 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.
Here are a few ideas for variations on the basic recipe:
Chop up some fresh basil (about ¼ cup) and mix it into the cream cheese.
Dried herbs are good too - you can mix ½ teaspoon of any dried herb (oregano and basil are especially good) into the cream cheese.
Like your food spicy? Add ¼ teaspoon red pepper flakes to the cream cheese.
Sprinkle a tablespoon of dry-grated parmesan cheese on top of the mozzarella for some added flavor.