Add the heavy cream, cream cheese, and butter to a medium sauce pan and cook over medium heat until the butter and cream cheese have melted.
Whisk in the ground mustard and pepper.
Remove from the heat and whisk in the cheddar and Gruyere until the mixture is smooth and creamy.
Serve immediately as the sauce will thicken as it cools.
Notes
Feel free to swap the Gruyere for extra cheddar or another cheese such as pepperjack, Parmesan, or gouda.To reheat, add a splash of cream and microwave in 30 second bursts, stirring in between, until the mixture is smooth, creamy, and hot. If your cheese sauce is too thick or too thin for your preferences, add additional cheese to thicken or additional cream to thin.