If using frozen spinach, thaw it and squeeze out any excess water.

If using fresh spinach, sauté it in a little olive oil until wilted, then squeeze out the liquid.

Set the prepared spinach aside.

Preheat your oven to 375°F (190°C).

In a medium bowl, combine the ricotta cheese, 3 oz of the grated Parmesan, and half of the mozzarella cheese.

Stir in the prepared spinach and season with a little salt and pepper to taste.

Spread ½ cup of marinara sauce evenly on the bottom of a 9x13-inch (or similar size) baking dish.

Layer lasagna sheets over the sauce, ensuring they cover the surface without overlapping.

Spread ⅓ of the spinach and ricotta mixture over the lasagna sheets, followed by another ½ cup of marinara sauce.

Repeat the process until you've assembled 3-4 layers, finishing with a final layer of lasagna sheets and the remaining marinara sauce.

Sprinkle the remaining Parmesan and mozzarella cheeses over the top of the assembled lasagna.

Seal the dish tightly with aluminum foil and place it in the oven to bake for 25 minutes.

Once the time is up, take off the foil and continue baking for another 15 to 20 minutes, or until the cheese on top is melted, bubbly, and has turned a lovely golden brown.

Allow the lasagna to sit for 10 minutes to set before cutting into portions and serving.

Enjoy this cheesy, spinach-filled goodness with a side salad or garlic bread!