Make the béchamel: Melt butter in a saucepan over medium heat. Add flour all at once and stir constantly for 1–2 minutes until a pale golden paste forms (roux). Add warm milk gradually in a thin stream, whisking continuously to prevent lumps. Cook, stirring, until the sauce thickens and coats the back of a spoon, 5–7 minutes. Season with salt and freshly grated nutmeg. Remove from heat.
Preheat oven to 355°F (180°C). Butter a baking dish approximately 8x8 inches (20x20 cm).
Assemble the lasagna: Spread a thin layer of béchamel on the bottom of the dish (this prevents sticking). Layer:
pasta sheets → bolognese → béchamel → Parmigiano. Repeat for a minimum of 3 layers. Finish the top layer
with béchamel and a generous amount of Parmigiano.
Cover tightly with foil. Bake 30 minutes.
Remove foil. Bake an additional 15 minutes until the top is golden and the edges are bubbling.
Rest 10-20 minutes before cutting. This is non-negotiable: cutting immediately will cause the layers to slide apart.