01 -
Preheat your oven to 200°C (400°F) with the fan on.
02 -
In a blender, finely chop the onion.
03 -
In a saucepan, melt 50g of butter over medium heat. Add 50g of flour and cook for 1-2 minutes, stirring constantly to create a smooth roux.
04 -
Gradually whisk in 750ml of milk, ensuring there are no lumps. Add the chopped onion, egg yolk, salt, pepper, and nutmeg.
05 -
Cook the sauce for 8 minutes, stirring continuously, until it thickens.
06 -
Remove the saucepan from heat and stir in 100g of the grated Emmental cheese until fully melted and smooth.
07 -
Lightly butter a baking dish. Spread a thin layer of Mornay sauce on the bottom.
08 -
Arrange a layer of lasagna sheets over the sauce, trimming them to fit if necessary.
09 -
Spread one-quarter of the Mornay sauce over the pasta, then top with two slices of ham and one-third of the sliced Raclette cheese. Repeat these layers of pasta, sauce, ham, and Raclette cheese twice more.
10 -
Top with a final layer of pasta sheets, the remaining Mornay sauce, the remaining ham slices, and the remaining Raclette cheese.
11 -
Sprinkle the remaining 50g of grated Emmental cheese evenly over the top.
12 -
Bake in the preheated oven for 20 minutes, or until the top is golden brown and bubbly.
13 -
Let the lasagna cool slightly before portioning and serving.