Squeeze thawed spinach between several layers of paper towels to remove as much excess moisture as possible. Transfer spinach to a medium bowl and stir in ricotta cheese, egg, salt, pepper, and garlic powder until well combined.
Pour 1 cup marinara sauce in the bottom of the prepared baking dish. Arrange 1/2 of the ravioli in a single layer over the sauce. Pour 1 cup sauce over the ravioli, then spread ricotta mixture over the sauce and top with 1/2 cup mozzarella cheese. Top with remaining ravioli, remaining sauce, and remaining mozzarella.
Cover with parchment paper and then cover tightly with foil. Freeze for up to one month.
When ready to cook, preheat the oven to 350 degrees F (175 degrees C).
Place frozen covered lasagna in the preheated oven and bake for 1 1/2 hours. Uncover and bake until golden and bubbly, about 20 more minutes. Let stand for 10 minutes before serving.