• Preheat oven to 425°F with a rack in the center.

For the Filling

• Finely chop the portobellos (into about ¼-inch dice), or roughly chop them with a knife and then pulse briefly in a food processor.
• Melt 2 tablespoons of the butter in a large non-stick skillet over medium-high heat.
• Add the portobellos and ½ teaspoon of the salt. Sauté until they've released their moisture and it has cooked off, and the mushrooms are beginning to brown.
• Add the onion and garlic and reduce the heat to medium. Sauté, stirring occasionally, until the onions and garlic have softened a bit but not browned.
• Meanwhile, cook the spinach according to package directions. Drain in a colander and let it sit until it is cool enough to handle. Pick it up by the handful and squeeze out as much liquid as you can.
• Crack the eggs into a large bowl and beat. Add the ricotta, about half of the parmesan cheese, ⅛ teaspoon nutmeg, ½ teaspoon salt, and ¼ teaspoon pepper. Mix thoroughly.
• Stir in the drained spinach until well distributed. Then stir in the portobello mixture.

For the Béchamel Sauce

• Melt the remaining 4 tablespoons butter in a medium, heavy-bottomed pot over medium heat.
• Add the flour all at once and cook, stirring constantly, for one minute.
• Add the milk, raise the heat to high, and bring to a boil, stirring almost constantly.
• When the mixture boils, reduce the heat so it is simmering vigorously and cook for ten minutes or so, stirring occasionally, until the sauce is thick enough to coat the back of your spoon.
• Turn off the heat and add about half of the remaining parmesan cheese, the remaining ⅛ teaspoon nutmeg, ½ teaspoon salt, and ¼ teaspoon pepper.

To Assemble and Bake the Lasagna

• Begin with a deep 9×13-inch baking pan. It needs to be at least 2 ½ inches deep — the shallower 2-inch depth will not accommodate all the ingredients.
• Spread a thin layer of the béchamel to coat the bottom of the pan.
• Layer ⅓ of the lasagna noodles over the sauce. Top the noodles with half of the ricotta filling mixture.
• Repeat with another layer of béchamel, noodles, and ricotta filling.
• For the last layer, put the remaining ⅓ of the noodles directly over the second layer of ricotta filling, pour the remaining béchamel on top, and sprinkle with the remaining parmesan cheese.
• Cover the pan with foil and set it over a rimmed baking sheet to catch any drips. Bake for 30 minutes.
• Remove foil and bake for an additional 10 minutes to brown the top a bit.
• Rest lasagna for 10 minutes or so before serving. It's great alone or with marinara sauce to pass at the table.