Pour a little or a lot over your favorite meat, veggies or even eggs. My ultimate favorite is lashings of mustard sauce poured over pennysmenu Corned Siverside.
In a small pot, gently melt the butter and add the flour.
On a low heat stir the melted butter and flour for two minutes without allowing to brown, to cook out the flour. * Don't cut this step short because the sauce will have a raw after taste if you do.
Next add about half a cup of the milk and stir until the sauce starts to thicken. ***See Notes
As the sauce thickens, gradually add the remaining milk stirring constantly. I usually add around half a cup at a time, reserving a little milk towards the end of cooking to adjust the consistency.
When the sauce is almost at the desired pouring consistency, add the mustard and stir to combine.
Continue to simmer very gently for a minute or two then its ready to pour into a gravy boat or sauce jug and serve.
I was taught that if you are using cold milk for a sauce, to add it all at once and if you are using warm milk, to add it gradually. I have had more success adding the milk gradually whether it is warm or cold. If a few lumps develop while cooking, a quick mix with an electric or hand beater usually removes them.