Heat the bechamel sauce in a medium-size pot over low heat until hot.
Add in the cheese and whisk until it is completely mixed in and smooth.
Finish by whisking un unsalted butter until melted in and adjust seasonings with salt and pepper if it needs it.
Serve.
Notes
Low and slow is the name of the game when it comes to classic Mornay sauce. In other words, maintain a gentle heat (low heat is ideal) on the stove and gradually melt the cheese into the sauce to ensure a silky, lump-free consistency. Trust me, the few extra minutes are so worth it.Don’t stop whisking: Keep the sauce moving as you incorporate each ingredient to help emulsify the sauce and prevent it from sticking to the bottom of the pan.Strain if it’s lumpy: If your sauce turns out lumpy or grainy, just pass it through a fine-mesh strainer or chinois to smooth the consistency.Thin the sauce if needed: If it feels too thick or isn’t pourable, whisk in a splash of hot milk until it’s thinned to your liking. Start with about ¼ cup and go from there.Make-Ahead: You can prepare mornay sauce up to 2 days ahead and keep it covered in the refrigerator until you’re ready to use it. How to Store: Let the sauce cool to room temperature, then transfer it to an airtight container and refrigerate it for up to 4 days. How to Reheat: Warm the sauce slowly in a saucepan over low heat, whisking frequently to bring back its silky texture. If the consistency is too thick after chilling, stir in a splash of warm milk to loosen it up. Taste and adjust the seasonings if needed.