Melt butter until it starts browning then add flour and stir consistently to make the "roux"

You can now add milk gradually to the "roux" (using a whisk)
Keep whisking at all time under medium to low heat

Once the mixture starts to thicken and look homogenous (after a couple of minutes) add the rest of the ingredients of the heat (use a spatula to avoid grated cheese clumping up)
Set aside or use immediately if desired or store in the fridge and use when cold