Crush in a bowl with a wooden spoon about a teaspoonful of mustard flour, half an average saltspoonful each of salt and white pepper, then work into it the yolk of a raw egg (see that a special wooden spoon is kept for mayonnaise making, and that it is well scalded each time of using), and add to it, drop by drop, the very best salad oil, till it is quite as thick as butter; then add about 1½ tea-spoonfuls of vinegar (plain or flavoured, as you please), when it should become quite creamy, and is ready. It should be used at once, but if it must stand a little, stir into the above quantity about a small dessertspoonful of actually boiling water. The above will make half a pint of sauce. If from any cause, such as the too quick pouring in of the oil, etc., the sauce thin or curdle suddenly, the only thing to do is to break another egg into a clean basin and slowly work into it the curdled sauce, almost drop by drop. If stored in a cool place it is possible to keep mayonnaise good for a day or so, but it must be closely covered up, for exposure to the air soon turns it oily and rank.
Mayonnaise, like many other sauces, can be very much varied. For instance, white mayonnaise is prepared by using lemon juice instead of vinegar, and adding at the last a gill of very stiffly whipped cream to the mixture. For red mayonnaise stir equal parts of mayonnaise and tomato puree together, adding, if necessary, a dash of flavoured vinegar to the mixture. For green mayonnaise blanch some sprays of parsley, chervil, and tarragon, drain oft the water, pound the herbs smoothly, and then incorporate them with some mayonnaise sauce, adding a drop or two of green colouring to get it all a delicate green. If mayonnaise aspic is desired, add three large tablespoonfuls of mayonnaise to a short half pint of aspic jelly, incorporating them well. A delicious masking sauce may be made by stirring a full tablespoonful of mixed olives, or truffles, as you please, into a short half pint of mayonnaise, with a dust of coralline pepper, and a squeeze of lemon juice, stirring in at the last about a gill of stiffly whipped cream.
All these sauces should be stood on ice till wanted.