From "Slow Cooker Revolution," by the editors of Cook's Illustrated (America's Test Kitchen). Do not use no-boil noodles, they will turn too soft. And do not use homemade cream sauce, or it will curdle. Store-bought Alfredo sauce will stay intact. Serves 6 to 8.
Ingredients
Vegetable oil spray
8 curly-edged lasagna noodles (7 ounces), broken in half
1 tablespoon extra-virgin olive oil
1 1/2 pounds white mushrooms, trimmed and sliced thin
2 garlic cloves, minced
20 ounces frozen spinach, thawed, squeezed dry and chopped coarse
1 jar (15 ounces) Alfredo sauce
1 3/4 cups ricotta cheese (15 ounces)
1 1/4 cups grated Parmesan cheese (2 1/2 ounces)
1/2 cup minced fresh basil
1 large egg
4 cups shredded mozzarella cheese (1 pound)
Directions
Line slow cooker with aluminum foil collar: Layer and fold sheets of heavy-duty foil until you have a six-layered rectangle that measures 16 by 4 inches. Press the collar into the back side of the slow cooker insert. Fit two more large sheets of foil into the slow cooker, perpendicular to each other, with the extra hanging over the edges of the cooker for a sling to help remove the lasagna later.
Bring 4 quarts water to boil in large pot. Add broken lasagna noodles and 1 tablespoon salt and cook, stirring often, until al dente. Drain noodles, rinse under cold water until cool, then spread out in single layer over clean kitchen towels and let dry. (Do not use paper towels; they will stick to noodles.)
Heat oil in pot over medium-high heat until shimmering. Add mushrooms, garlic, and 1/4 teaspoon salt, cover, and cook until mushrooms are softened, about 5 minutes. Uncover, and continue to cook until mushrooms are dry and browned, 5 to 10 minutes longer. Stir in spinach and Alfredo sauce, scraping up any browned bits, and season with salt and pepper to taste.
In bowl, mix ricotta, 1 cup Parmesan, basil, egg, 1/2 teaspoon salt, and 1/2 teaspoon pepper together. Spread 1/2 cup mushroom-spinach sauce into prepared slow cooker.
Arrange 4 lasagna noodle pieces in slow cooker, overlapping if necessary, then dollop 9 rounded tablespoons of ricotta mixture over noodles. Sprinkle with 1 cup mozzarella, then spoon 1 cup more mushroom-spinach sauce over top. Repeat layering of lasagna noodles, ricotta mixture, mozzarella and mushroom-spinach sauce twice more.
For final layer, arrange remaining 4 noodles in slow cooker, then top with remaining mushroom-spinach sauce and sprinkle with remaining mozzarella and remaining Parmesan. Cover and cook until lasagna is heated through, about 4 hours on low.
Let lasagna cool for 20 minutes. Using sling, transfer lasagna to serving platter and serve.