Melt the butter in a saucepan over medium heat until foaming.
Add the flour and stir quickly to eliminate lumps.
Remove from heat and add the milk in a continuous stream whilst whisking.
Return to the heat ans stir continuously with a spoon until it boils and thickens enough to coat the back of the spoon.
Stir in the cheese until fully melted. Add the nutmeg and stir through.
Lasagne
Preheat your oven to 180 degrees.
I use an 8x 13 inch rectangular cast iron dish for my lasagne, you will need to adjust your lasagne to fit the dish that you have.
Start with a ladle of bolognaise sauce. Put it in the bottom of the dish. This helps to cook the bottom layer as the pasta sheets require liquid to cook. It also helps with removing it from the dish. Don't ignore this step, it is essential.
Top with as many sheets of pasta as you need, my dish needs 4 sheets per layer. It doesn't matter if they overlap.
Top with a layer of sauce (about 4 ladles).
Top with lasagna sheets.
Top with⅓ of the bechamel.
Repeat, ending with bechamel sauce.
Top with grated cheese, I use a mix of mozzarella and cheddar.
Cover with foil and bake for 20 minutes.
Remove the foil and bake another 20 minutes or until golden and bubbling.