Preheat the oven to 375°F (190°C). Grease a 9×13-inch baking dish lightly with olive oil or nonstick spray.

Cook lasagna noodles according to package directions until al dente. Drain and lay them flat on a parchment sheet or lightly oiled surface to prevent sticking.

Heat 1 tablespoon olive oil in a skillet over medium heat. Add the diced onion and cook until translucent, about 4–5 minutes. Add garlic and cook for 30–60 seconds until fragrant.

Add fresh spinach in batches and cook until wilted, tossing as it cooks (about 2–3 minutes). If using frozen spinach, squeeze excess water out with a towel or cheesecloth. Season with salt and pepper.

Transfer the cooked spinach mixture to a bowl and let cool slightly. Stir in the ricotta, taste and adjust seasoning with salt and pepper. Optionally add a pinch of nutmeg.

Spread a thin layer of white sauce (about 1/2 cup) on the bottom of the prepared baking dish to prevent sticking.

Layer a single layer of lasagna noodles, spread half of the spinach-ricotta mixture over, top with shredded mozzarella, and spoon on more white sauce.

Repeat layering noodles, spinach-ricotta, mozzarella, and sauce until ingredients are used, finishing with a final layer of sauce and remaining mozzarella.

Sprinkle grated Parmesan evenly across the top. Cover the dish tightly with foil.

Bake covered for 25 minutes, remove the foil, and bake uncovered for an additional 15 minutes or until the top is golden and bubbling.

Remove from oven and let rest for 10–15 minutes before slicing.