With the pulp of two three-ounce onions that have been simmered in milk till tender and passed through the sieve, when worked into half a pint of sauce blonde, with a delicate seasoning of salt and spiced pepper, you have onion sauce.
For sauce soubise the process is slightly different:—Blanch the two onions for five minutes, then cut them up into a small mince; melt an ounce of butter in a saucepan, put in the mince, stir over a low fire till the onions seem about to colour, then take off the saucepan and dilute with half a pint of sauce blonde; boil, skim, reduce a little over the fire again, and then pass the whole through the sieve, putting the purée into the saucepan again to be reheated in the bain-marie.