Take a pan add butter once butter melts add flour mix well with out forming lumps do not brown this mixture then switch off the flame.
Take half onion stud cloves into onion and bay leaf keep a side.
take warm milk add little by little to the flour mixture mix with out forming lumps once it is done now switch off the flame keep mixing this mixture till it comes to boil in a very slow flame,add salt,pepper, the onion and boil for 10 min, to check the sauce is ready take the spoon just draw a line at the back it should hold to that line.
once the sauce is ready remove the onion switch off the flame, transfer to another bowl.
can use this sauce for pastas.