Preparation

• Thaw frozen spinach and squeeze out excess liquid. My favorite method of doing this is placing spinach in a round metal steamer basket, closing the sides and squeezing water out or using a cheesecloth. It’s much more efficient than using paper towels.
• Cook the lasagna noodles: Cook lasagna noodles until al dente. Drain, rinse under cold water and lay flat on a few sheets of parchment paper. Drizzle with a little olive oil to make them easier to work with when you assemble later on.
• Saute the onions and mushrooms: In a large nonstick skillet over medium-high heat, melt the butter. Add in the onions, salt and pepper. Saute 2-3 minutes, stirring occasionally. Next, add in the garlic and cook for an additional minute. Then add in the chopped mushrooms and Italian seasoning. Cook until the mushrooms cook down and liquid has evaporated, about 7-10 minutes, stirring occasionally. Remove from heat, stir in the drained spinach and a little more salt and pepper, to taste. Set aside to cool.
• Combine ingredients for ricotta mixture: In a large bowl, combine the whole milk ricotta, Parmesan cheese, half of the shredded mozzarella, eggs and parsley. Once spinach mixture has cooled, add ricotta mixture to spinach/mushroom mixture and combine.
• Make the white sauce sauce next: In a large heavy bottomed saucepan, melt the butter over medium heat. Add in the flour and whisk to make a roux, about 1 minute. Slowly pour in milk, whisking continually between each addition. Continue to slowly drizzle in milk and whisk until all milk is added. Whisk over medium-heat until mixture comes to a boil, it will begin to thicken. Remove from heat and stir in the Parmesan cheese, nutmeg, salt and pepper.
• Preheat the oven to 375°F.
• Pour about ½ cup white sauce in the bottom of a 9x13-inch oven safe pan. (Or enough sauce to coat the bottom).
• Add a layer of lasagna noodles on top in an even layer, tearing them as needed to fit the pan.
• Dollop ⅓rd of the spinach/mushroom/ricotta mixture over the noodles and spread in an even layer. Followed by ½ cup of white sauce. Arrange another layer of noodles on top and repeat process 2 more times.
• Arrange one final layer of noodles on top, the last of the white sauce and sprinkle the top with remaining mozzarella cheese.