This recipe is adapted from The Lady's Companion, a cookbook published in 1753, which was owned by Martha Washington.
2 large Bermuda onions, thinly sliced
1 quart milk
1 egg yolk
1/2 teaspoon mace blades
1 1/2 teaspoons salt
1/2 cup (1 stick) butter Croutons
Place onions, milk, mace, butter, and salt in a saucepan. Bring to a boil, then reduce heat and cook slowly for 30 to 40 minutes or until onions are very tender. Pick out mace blade and discard. Beat egg yolk in a small bowl, then add a little of the hot soup, beating constantly. Pour egg mixture into soup and cook a minute or two to thicken slightly. Sprinkle each serving with finely chopped parsley, then add a few croutons. Serves 4.
To make toasted croutons: Toast old firm bread, then cut into tiny squares.
Hot soup at table is very vulgar; it either leads to an unseemly mode of taking it, or keeps people waiting too long whilst it cools. Soup should be brought to table only moderately warm.
—Charles Day, Hints on Etiquette, 1844