Make a thick béchamel sauce with the butter, flour, and milk. Separate the eggs, placing the whites and yolks in different bowls. Add the yolks to the béchamel and keep stirring, then add the Swiss cheese. Remove from heat. Season with salt and pepper.

Beat the egg whites until stiff peaks form, then gently fold them into the mixture. Butter a deep, oven-safe dish and pour in the mixture.

Bake for 20 minutes at 350°F (180°C). Serve immediately out of the oven with a salad, for example. (Don’t wait, as the soufflé may collapse).