Pour the milk into a microwave-safe measuring cup and microwave it for 1-2 minutes. (You don’t want the milk to boil, just warm it up so it isn’t cold. This will help create a smooth sauce!)
In a saucepan, melt the butter over medium heat.
Whisk in the gluten-free flour, continue whisking for 2-3 minutes. (To cook off the raw flour taste.)
Slowly pour in the warm milk, whisking constantly and vigorously to avoid creating any lumps.
Bring to a gentle simmer and cook for a few more minutes until thickened, whisking occasionally. (This will take about 5 minutes.)