2½ tablespoons butter
1½ tablespoons flour
1 cup heavy whipping cream or ½ cup whole milk, hot
1 cup cream cheese
½ cup grated cheddar cheese
⅓ cup grated mozzarella cheese
½ teaspoon mustard (optional)
1½–2 pinches of salt
Melt the butter in a saucepan. Add the flour and whisk together over medium heat for 2–3 minutes, to make a white roux. Do not allow to brown. Remove from heat and allow to cool nearly to room temperature.
2–3 minutes
Gradually add hot heavy whipping cream to the roux, whisking to combine. This will also work with a hot roux and room temperature heavy cream or milk.
Add the salt, cheese, and mustard, then heat till just simmering and cheese has melted.