Preheat the oven to 350*. Melt butter in a pan on medium heat, whisk in the flour and keep stirring until smooth. Add in the milk/cream, continue stirring, and bring to a low boil for a few minutes, being careful not to let it burn. (You're making a basic white sauce here, or a roux.) Lower the heat and stir in the cheese as it melts, along with the sea salt
and pepper. Move the pan away from the heat. Beat just the egg yolks in a separate bowl. Stir about 1/2 cup of the hot cheese sauce into the yolks, and then stir all of the yolk mixture back into the pan, continuing to stir it all together. Stir in any optional ingredients. In a separate bowl, beat the egg whites until they thicken (I use my hand blender
for this), and then fold this into the egg and cheese mixture.