Heat the oven to 400°F. Meanwhile, arrange 12 dried lasagna noodles in an even layer in a 9x13-inch baking dish. Cover the noodles with hot tap water and let soak while you prepare the rest of the ingredients, about 30 minutes.

If using frozen spinach, thaw 15 to 16 ounces according to package directions. Transfer to a clean kitchen towel, roll it up in the towel, and squeeze out all the excess moisture.

Stir 1 (about 15-ounce) container ricotta cheese, 1 ounce of the finely grated Parmesan cheese, ¼ teaspoon of the kosher salt, and ¼ teaspoon of the black pepper together in a medium bowl.

Heat 2 tablespoons olive oil in a large pot or Dutch oven over medium-high heat until shimmering. Add 1 pound thinly sliced cremini mushrooms mushrooms and cook, stirring occasionally, until the mushrooms have released their liquid, the liquid has evaporated, and the mushrooms have browned, 14 to 16 minutes.

Reduce the heat to medium. Add 1 finely chopped medium yellow onion, ½ teaspoon of the kosher salt, and the remaining ¼ teaspoon black pepper. Cook, stirring occasionally, until the onion is translucent, about 4 minutes. Stir in 3 minced garlic cloves and 1 teaspoon dried Italian seasoning, and cook until the garlic is fragrant, about 1 minute.

If using fresh baby spinach, add 1 pound a few handfuls at a time and stir until just wilted, about 5 minutes. If using frozen spinach, add to pot and cook, stirring occasionally, until heated through, about 1 minute. Transfer to a plate.

Melt ½ stick unsalted butter in the same pot (no need to clean) over medium heat. Add ⅓ cup all-purpose flour and cook, whisking constantly, until bubbling and it begins to dry out slightly, about 2 minutes. Gradually whisk in 4 cups whole milk until smooth. Bring to a boil over high heat.

Reduce the heat to maintain a simmer and cook, stirring occasionally, until thickened, about 2 minutes. Stir in the remaining 2 teaspoons kosher salt and 1 pinch ground nutmeg. Remove the pot from the heat.

Drain the lasagna noodles and wipe the baking dish dry. If there is any accumulated liquid on the plate of spinach and mushrooms, pour it off.

Spread ¼ cup béchamel sauce in the baking dish into a thin, even layer. Arrange 4 of the lasagna noodles in an even layer on top of the sauce, breaking them if needed to fit (it’s OK if the noodles overlap slightly). Dollop and spread ½ of the ricotta mixture (about 1 cup) over the noodles. Spoon ½ of the mushroom and spinach mixture (about 2 cups) over the ricotta. Sprinkle with 1 cup of the shredded mozzarella cheese. Dollop and spread about 1 heaping cup of the béchamel sauce over the mozzarella.

Repeat layering 4 more lasagna noodles, the remaining ricotta mixture, the remaining mushroom and spinach mixture, 1 cup of the mozzarella, and 1 heaping cup of the béchamel sauce. Top with the remaining 4 lasagna noodles, remaining béchamel sauce, remaining mozzarella, and remaining 1 ounce grated Parmesan.

Bake uncovered until the cheeses are melted and browned, and the sauce is bubbling, 30 to 35 minutes. Let cool for at least 10 minutes before serving. Garnish with red pepper flakes and torn fresh basil leaves if desired.