1. Cook the pasta in boiling salted water. Remove 1 minute before the end of cooking, they will finish cooking in the oven. Set aside.
2. Melt the butter then add the flour. Heat while whisking until you obtain a white roux.
3. Preheat your oven to 220°C (428°F) and place your dish on a perforated baking sheet.
Spread the pasta in the dish and place the chicken on top. Grate 100g of Cheddar and 50g of Emmental with a grater, then cover the chicken with it. Pour the béchamel sauce over it.
4. Slice the onions using a mandoline and sweat them in butter with a pinch of salt until they have a light color. Add them to the cooked pasta. Set aside.
5. Add the milk, mustard, grated nutmeg, and pepper. Whisk until you obtain the béchamel sauce.
Incorporate 1/3 of the béchamel sauce into the pasta and mix everything together.
6. Mix the breadcrumbs with 30g of grated Emmental and spread them over the béchamel sauce. Bake for 15 minutes at 220°C (428°F) until golden brown.