Instructions

• Preheat oven to 375°F (190°C).
• If using the sausage, slice the links and brown them in a skillet over medium heat until cooked through; transfer to a plate and set aside.
• Place all three packages (3 × 10 oz) frozen chopped spinach in a large microwave-safe bowl and cover. Microwave until completely thawed, about 15 minutes, stirring once halfway through.
• After thawing, pour off and reserve 1/3 cup of the spinach liquid into a small measuring cup. Squeeze the thawed spinach dry with your hands or a towel to remove excess moisture.
• Transfer the squeezed spinach to a food processor and pulse 8–10 times, scraping down the bowl as needed, until the spinach is finely ground. Transfer the processed spinach to a clean bowl. Measure out 1/2 cup of the processed spinach and set it aside for the sauce; keep the remaining processed spinach in the bowl.
• Make the tomato-spinach sauce: heat 2 tablespoons olive oil in a large saucepan over medium heat until shimmering. Add the finely chopped onion and cook until softened, about 5 minutes. Stir in the minced garlic and 1/8 teaspoon crushed red pepper flakes and cook until fragrant, about 30 seconds. Add the two 28-ounce cans crushed tomatoes, the reserved 1/2 cup processed spinach, 1 teaspoon kosher salt, and 1/2 teaspoon freshly ground black pepper. Cook, stirring occasionally, until the sauce is slightly thickened, about 10 minutes. Remove from heat and stir in 3 tablespoons chopped fresh basil. Set the sauce aside.
• Make the ricotta-spinach filling: wipe out the food processor and add 3 cups (24 ounces) ricotta and the reserved 1/3 cup spinach liquid; process until smooth, about 30 seconds. Add 1 1/2 cups grated Parmesan, the remaining 3 tablespoons chopped fresh basil, 2 large eggs, 1 1/2 teaspoons kosher salt, and 1/2 teaspoon freshly ground black pepper; process until combined. Transfer this ricotta mixture into the bowl with the remaining processed spinach and stir until evenly combined.
• Assemble the lasagna in a 13x9-inch baking dish: spread 1 1/4 cups of the sauce evenly over the bottom of the dish. Arrange 3 no‑boil lasagna noodles in a single layer over the sauce. Spread one-third of the ricotta-spinach mixture evenly over the noodles. Sprinkle 2/3 cup shredded mozzarella over the ricotta. Spoon 1 1/4 cups of sauce over the mozzarella and, if using sausage, scatter some of the browned sausage pieces over this sauce.
• Repeat the layer described in step 8 two more times: for each repetition place 3 noodles, spread the next one-third of the ricotta-spinach mixture, sprinkle 2/3 cup mozzarella, and top with 1 1/4 cups sauce (and more sausage if using). This creates three stacked noodle/filling layers.
• Finish the top: arrange the remaining 3 noodles over the last sauce, spoon the remaining sauce evenly over the top noodles, and sprinkle with the remaining 1 cup shredded mozzarella.
• Cover the pan tightly with aluminum foil and bake until bubbling around the edges, about 40 minutes. Remove the foil and bake until the cheese is melted and lightly browned, about 10 more minutes. Remove from the oven and cool on a wire rack for 30 minutes before slicing and serving.