In a medium saucepan, melt the butter over medium heat.
Whisk in the flour and cook for about 1 minute, until smooth and lightly bubbling (do not brown).
Slowly whisk in the warm heavy cream until completely smooth.
Cook for 2–3 minutes, stirring constantly, until slightly thickened.
Remove from heat. Add the cheeses gradually, stirring until fully melted and smooth.
Season with a pinch of salt. Taste and adjust as needed.
Serve warm.