Step 1

Thaw frozen spinach, if necessary. If using frozen spinach, thaw 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.

Step 2

Soak the lasagna noodles. Arrange a rack in the middle of the oven and heat the oven to 400°F. Meanwhile, arrange 15 dry 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 spinach and cheeses, about 20 minutes.

Step 3

Prepare the aromatics and cheeses. Dice 1 medium yellow onion. Mince 3 garlic cloves. Grate 12 ounces low-moisture mozzarella cheese on the large holes of a box grater (about 3 cups). Finely grate 2 ounces Parmesan cheese (about 1 packed cup).

Step 4

Sauté the aromatics and spinach. Heat 2 tablespoons olive oil in a large Dutch oven or pot over medium heat until shimmering. Add the onion and cook, stirring occasionally, until softened and translucent, 5 to 7 minutes. Add the garlic and sauté until 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 the liquid has evaporated, about 2 minutes. Season with 1/2 teaspoon of the kosher salt, 1/4 teaspoon of the black pepper, and 1/4 teaspoon red pepper flakes, if using. Remove from the heat and let cool for 5 minutes.

Step 5

Make the ricotta mixture. Meanwhile, place half of the Parmesan cheese, 1 container ricotta cheese, the remaining 1/4 teaspoon kosher salt, and remaining 1/4 teaspoon black pepper in a medium bowl and stir to combine.

Step 6

Begin to assemble the lasagna. Drain the lasagna noodles and set aside. Spread a thin layer of marinara sauce from a 32-ounce jar into the baking dish.

Step 7

Layer the lasagna. Arrange 5 lasagna noodles in an even layer on top of the sauce, breaking them if needed to fit (it’s okay if the noodles overlap slightly). Dollop and spread half of the ricotta mixture over the noodles. Spoon half of the spinach over the ricotta, then sprinkle with 1 cup of the mozzarella. Dollop and spread about a third of the marinara sauce (about 1 heaping cup) over the mozzarella.

Step 8

Finish layering the lasagna. Arrange 5 more noodles over the mozzarella, followed by the remaining ricotta mixture, remaining spinach, 1 cup of the mozzarella, and 1 heaping cup of the marinara sauce. Top with the remaining 5 noodles and the remaining sauce, spreading the sauce thin so that it almost completely covers the noodles. Sprinkle with the remaining 1 cup mozzarella and Parmesan.

Step 9

Bake the lasagna. Bake uncovered until the cheeses are melted and lightly browned, and the sauce is bubbling, about 30 minutes. Let cool on a wire rack for at least 15 minutes before serving.