Begin by melting the butter in a non-stick sauce pan. I have mentioned the onions as optional cause depending on the situation (i.e if it's going to be only for my little miss, the onion is skipped). But if you do add the onions, add it now. Make sure its finely chopped.
Once the onions have softened, add the flour and salt. Have the heat on medium low and stir constantly until the flour mixture goes pale and has a slightly nutty aroma. This takes around 5mins.
Next add the milk and whisk continuously till its smooth.
Reduce the heat to low and whisk occasionally until the sauce starts to thicken. Once its done (7-10mins later), set aside until it's ready to use
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