Preheat oven to 350 degrees F. Lightly spray a square 8-inch baking dish with olive oil cooking spray.
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, the onion and 1/2 teaspoon kosher salt. Cook, using a wooden spoon to stir and to break up the ground beef into small chunks, until onion is soft and meat is browned, 5-7 minutes.
Drain the beef mixture, then return to the skillet. Add the garlic and cook, stirring, 30 seconds, then mix in the marinara sauce. Turn heat off.
Place the cream cheese in a medium microwave-safe bowl and heat for 40 seconds to soften. With a fork, stir until smooth and creamy. Add the egg, the black pepper and the remaining 1/2 teaspoon kosher salt and use a hand whisk to whisk until smooth. With a spatula, add the drained spinach and mix to blend everything together.
Spread the spinach mixture evenly in the bottom of the prepared baking dish. Top with the meat mixture. Sprinkle the mozzarella and the Parmesan on top. Bake 30 minutes, until cheese is bubbly and lightly browned. Let stand ten minutes before cutting and serving.