Remove the saucepan from the heat and let it cool slightly for ~1-2 minutes. Stir in the grated Gruyère in small batches until it is completely melted and the sauce is smooth.
Tip: If you add cheese when the sauce is too hot, it can cause the sauce to “break”. This means the proteins and fat of the cheese will separate and the sauce will become grainy or oily instead of smooth and creamy