Thaw spinach and squeeze out moisture using a steamer basket or cheesecloth.

Cook lasagna noodles until al dente. Rinse and lay flat on parchment paper with a drizzle of olive oil.

In a skillet, melt butter over medium heat. Sauté onions with salt and pepper for 2–3 minutes. Add garlic, then mushrooms and Italian seasoning. Cook until moisture evaporates, 7–10 minutes. Stir in drained spinach. Let cool.

In a bowl, mix ricotta, Parmesan, eggs, half the mozzarella, and parsley. Combine with cooled mushroom-spinach mixture.

For the béchamel: melt butter in a saucepan, whisk in flour to form a roux. Gradually whisk in milk until thickened. Remove from heat, stir in Parmesan, nutmeg, salt, and pepper.

Preheat oven to 375°F.

Spread ½ cup white sauce in a 9×13-inch pan. Layer noodles, 1/3 ricotta mixture, ½ cup white sauce. Repeat layers 2 more times.

Top with final noodles, remaining white sauce, and mozzarella. Cover with greased foil.

Bake 30 minutes covered, then 10 minutes uncovered until bubbly and golden. Broil briefly if needed.

Cool for 15 minutes before serving.