In a medium saucepan, melt the butter over medium heat.

Add the flour to the melted butter, stirring constantly to create a roux. Cook for about 1-2 minutes until it becomes a light golden color.

Gradually whisk in the milk to the roux, ensuring there are no lumps. Continue to stir until the mixture thickens, usually 3-5 minutes.

Reduce the heat to low, and gradually add the shredded cheddar, Gruyere, and mozzarella cheeses to the sauce. Stir continuously until the cheeses are fully melted and the sauce is smooth.

Stir in the Dijon mustard and Marukan Organic Apple Cider Vinegar.

Season the sauce with salt and pepper to taste. If you like a bit of heat, add a pinch of cayenne pepper.

Continue to stir the sauce over low heat until it reaches your desired consistency. If the sauce becomes too thick, you can add a little more milk.