In a saucepan over medium heat, melt the butter. Whisk in the floor and cook for 1-2 minutes.

Slowly pour in the milk, whisking constantly until no lumps remain. Bring to a simmer until the sauce begins to thicken.

Slowly add in your cheeses in 3-4 batches, whisking after each one and not adding any more until the previous addition is fully melted and incorporated.

Season to taste with salt and pepper. Serve immediately.