01 -
Preheat your oven to 350°F. Lightly grease a square 8-inch baking dish or another 2-quart baking dish.
02 -
Defrost the spinach in the microwave according to the package directions. 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.
03 -
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.
04 -
Drain the beef mixture and return it to the skillet. Add the garlic and cook, stirring, for 30 seconds, then mix in the marinara sauce. Turn the heat off.
05 -
Place the cream cheese in a medium microwave-safe bowl and heat it for 40 seconds to soften. With a fork, stir it until smooth and creamy. Add the egg, black pepper, and remaining ½ teaspoon of kosher salt. Keep stirring the mixture until very smooth. With a spatula, add the drained spinach and mix to blend everything together thoroughly.
06 -
Spread the spinach mixture evenly on the bottom of the prepared baking dish. Top it with the meat mixture, and sprinkle the mozzarella on top.
07 -
Bake the lasagna until the cheese is bubbly and lightly browned, about 30 minutes.
08 -
Let the lasagna stand for ten minutes before cutting and serving.