01 -
In a large skillet, heat olive oil over medium heat. Sauté diced onion and minced garlic for 2 to 3 minutes until fragrant and softened.
02 -
Add sliced mushrooms, salt, black pepper, and Italian seasoning to the skillet. Cook until mushrooms become tender and any liquid has evaporated, approximately 8 to 10 minutes. Stir in drained spinach, mix until evenly combined, then remove from heat.
03 -
In a medium saucepan, melt butter over medium heat. Add flour, whisking constantly, and cook for 1 minute to form a roux. Gradually whisk in milk and broth, stirring continuously. Cook until sauce thickens, about 5 to 7 minutes. If desired, incorporate nutmeg. Remove sauce from heat.
04 -
Lightly grease a 9x13-inch baking dish. Spread a thin layer of the prepared white sauce on the bottom. Place 3 lasagna noodles in a single layer over the sauce. Top with half of the ricotta, half of the spinach and mushroom mixture, a portion of mozzarella, and additional white sauce.
05 -
Repeat the layering using another 3 noodles, remaining ricotta, spinach-mushroom mixture, additional mozzarella, and more white sauce.
06 -
Lay the final 3 noodles on top. Spread the remaining sauce over the noodles and sprinkle evenly with the rest of the mozzarella and all Parmesan cheese.
07 -
Cover the baking dish with aluminum foil and bake at 375°F for 25 minutes. Uncover and continue baking for an additional 10 to 15 minutes until bubbling and golden brown on top. Cool slightly before serving.