Melt 40 g of butter in a saucepan over medium heat.
Add 40 g of flour all at once and whisk vigorously to form a roux.
Cook for 2 minutes on low heat, stirring constantly to avoid over-browning.
Remove the pan from heat and gradually pour in 450 ml of milk, whisking vigorously to avoid lumps.
Return to medium heat, whisking continuously until the sauce thickens.
Season with salt, freshly ground black pepper, and a pinch of grated nutmeg.
Off the heat, add 115 g of grated cheddar and mix until fully melted.