Prep the Ingredients:Thaw frozen spinach or boil fresh spinach for 1–2 minutes. Squeeze out all excess liquid. If using fresh mozzarella, slice it and place it on paper towels to absorb moisture. In a bowl, mix the spinach with ricotta cheese, ¾ of the parmesan, salt, and pepper.
Preheat and Layer:Preheat your oven to 400°F (200°C). Spread ½ cup of marinara sauce on the bottom of a baking dish. Add a layer of lasagna noodles. Spread another ½ cup of marinara sauce, then ¼ of the spinach-ricotta mixture, and top with mozzarella cheese. Repeat the layers until you have 4 full layers of noodles and filling.
Top and Bake:Finish with a final layer of noodles, marinara sauce, mozzarella, and the remaining parmesan. Bake for 25–30 minutes, until bubbling and golden on top.
Rest and Serve:Let the lasagna rest for 10 minutes before slicing. Garnish with fresh basil, if desired, and serve warm.