Step 1

In a small saucepan, melt butter over medium-high heat (do not allow it to brown). Add flour and whisk to form a paste. Continue to cook, stirring, until raw flour scent is gone, about 1 minute. Whisking constantly, add milk in a thin, steady stream, or in increments of a couple of tablespoons at a time, whisking thoroughly and getting into all corners of the pan to maintain a homogeneous texture. Sauce will initially become very thick, then get very thin once all the milk is added.

Step 2

Heat, stirring, until sauce comes to a simmer and begins to thicken slightly. Reduce heat to low and cook, stirring, until sauce is just thick enough to coat the back of a wooden spoon, about 3 minutes.

Step 3

Season with salt and pepper. Whisk in nutmeg. If any lumps form, simply whisk thoroughly to remove them and smooth out sauce, or use a hand blender or countertop blender if lumps are particularly large or tough. Use béchamel sauce right away, or press a piece of plastic wrap over surface of sauce to prevent a skin from forming and keep warm until ready to use. (Béchamel sauce can be cooled and stored in a sealed container in the refrigerator for several days. Reheat very gently on the stovetop or in a microwave to use.)

Step 4

If Using Homemade Pasta: Follow the recipe and instructions here through step 10 (you will end up with long sheets of fresh pasta), doubling the quantity to yield 2 pounds total (this will make more pasta than you will need, but it's best to have extra and avoid running out midway through assembly). Cut those sheets into roughly 8-inch-long rectangles.

Step 5

In a pot of salted boiling water, cook pasta, working in batches if necessary, until al dente (this will vary depending on the pasta you use, but for store-bought it's about 1 minute less than package instructions), then use a spider or mesh strainer to transfer immediately to a large ice bath to cool.

Step 6

Drain pasta well, then rub each sheet lightly on both sides with vegetable oil (to prevent sticking). You can hold the cooked pasta sheets for up to 3 hours, but if you're doing this, it's best to lay them out on a parchment-lined baking sheet, with layers of plastic wrap between the layers of pasta (once again, to prevent sticking).

Step 7

If Using No-Boil Pasta: Soak in warm water for 30 minutes to partially hydrate, then drain on paper towels or kitchen towels.

Step 8

To assemble and bake lasagna, preheat oven to 375°F (190°C). Grease a 9- by 13-inch baking dish with butter. Spoon a thin, even layer of ragù on bottom of baking dish, then lay down a layer of lasagna noodles; if's okay if they overlap somewhat, but you can cut any sheets that are too large to avoid excessive doubling up.

Step 9

Top pasta with another thin layer of ragù (thin enough that you can see the pasta through it in some spots). Drizzle a small amount of béchamel all over ragù, then top with a showering of grated Parmigiano-Reggiano. Repeat this layering process with pasta, ragù, béchamel, and grated cheese until baking dish is full; this should be about 6 layers. Finish with a top layer of pasta, then coat that with an even layer of the remaining béchamel sauce. Grate a final generous amount of cheese on top.

Step 10

Bake lasagna until bubbling and browned on top, about 35 minutes (placing a rimmed baking sheet under the baking dish is good insurance in case anything bubbles over). Let rest 10 minutes, then serve.