Heat the milk and heavy cream over medium heat in a saucepan until it steams.

Add the finely grated gruyere cheese to the saucepan and stir until it has completely melted and incorporated into the milk mixture.

Whisk together the egg yolks in a separate bowl until they are light and frothy.

Carefully pour the cheese mixture into the eggs while continuously whisking.

Add the mixture back to the pot and, on medium heat, stir until it has thickened like a crème anglaise.

Blend the whole mixture with a hand blender and strain through a fine sieve, adding and blending in the xanthan gum at this step if using.

Season to taste with salt and pepper.

Either serve immediately or refrigerate and serve cold.

Add the sauce to a siphon and charge it with one gas canister.

If the sauce is too thick, thin it with milk.