11. Build the Lasagna
In a 9- by 13-inch lasagna pan, spread a generous spoonful (about ½ cup) of the bechamel over the bottom of the pan and cover with lasagna noodles. Cover the noodles with ¾ cup bechamel, then dust with ¼ cup pecorino. Tile one third of the squash slices over the sauce, then sprinkle one third of the caramelized onions evenly over the top. Finally, dot the onions with a quarter of the mozzarella. Repeat this layering—noodles, bechamel, pecorino, squash, onions, mozzarella—twice more. For the final layer, cover the noodles with the last of the bechamel, pecorino, and mozzarella.