This classic baked manicotti dish from "The Geometry of Pasta," by Caz Hildebrand and Jacob Kenedy, bursts with creamy ricotta cheese.
Preheat oven to 425 degrees.
Bring a large pot of salted water to a boil. Add manicotti and cook, according to package directions, until al dente. Drain, reserving pasta cooking water; rinse with cold water and drain again. Set aside.
Bring pasta cooking water to a boil and add spinach; cook until just wilted. Drain, rinse with cold water, and drain again. Squeeze out excess water from spinach and finely chop.
Transfer spinach to a large bowl along with ricotta, half of the Parmesan, egg yolks, and nutmeg; season with salt and pepper and stir to combine. Stuff spinach mixture into manicotti and set aside.
Melt butter in a medium saucepan over medium-high heat. Add flour and stir until well combined. Gradually stir in milk; continue cooking, stirring constantly, until mixture is smooth and thickened.
Evenly spread tomato sauce on the bottom of a small rectangular baking dish. Top with an even layer of stuffed manicotti. Pour butter mixture over manicotti and sprinkle with remaining cheese. Transfer to oven and bake until top is browned, 20 to 25 minutes. Let stand 10 to 20 minutes before serving with additional Parmesan cheese.