Put oil and butter in a heated pan. (Butter, when added directly, tends to burn, hence added to oil)
When butter melts, add wheat flour.
Stirring continuously, cook on medium flame for a minute or so, It should be like a roux (paste). If the mix is even little crumbled, add little more butter.
While cooking it will be frothy, keep stirring.
When the colour of the flour changes a little, also you can feel the aroma coming out of it then switch off the flame.
Pour little milk with one hand and keep stirring with the other hand. You have to be quick otherwise lumps may form.(Always add milk in batches)
When mixed homogenously, again pour some more milk repeating the act of stirring. It will be a smooth paste.
Lastly, put the remaining milk.
Mix nicely.It will be very liquidy at this stage.
Cook this liquidy mixture on medium flame.
Soon it will start thickening.
After about 5 to 6 minutes, when it coats the back of the spoon and you get the desired sauce-like consistency, then switch off the flame.
Add salt, pepper and nutmeg. Use as desired.