1. Wash and cut the Swiss chard, then cook them for 10 minutes in a pressure cooker. Drain them and let them drain while you prepare your roux.
2. Make a blond roux with butter, Roquefort cheese, and 30g of cornstarch, over low heat.
3. Gradually add the milk to the roux to avoid lumps.
4. Thicken the mixture by stirring often but gently, and optionally add some pepper.
5. Add the egg yolk and grated Gruyère cheese, then turn off the heat.
6. Do not add salt!
7. Gently mix the Swiss chard with the mixture (béchamel), pour into a dish, and bake in a preheated oven at 180°C for about 25 minutes.