Cut up some mushrooms or cooked sweetbreads and place them in a sautoir with allemande sauce (No. 407), season, fill an omelet with either the mushrooms or the sweetbreads and finish by pouring the sauce around.
Ranhofer writes for a professional brigade with sauces already standing on the stove, so the whole dish here is one sentence. The work is in the two components he assumes you have: allemande sauce and cooked sweetbreads.
The Epicurean (1894) is Charles Ranhofer's record of thirty-odd years as chef at Delmonico's in New York, and this entry shows how the book works: a numbered dish that is really an instruction to combine two other numbered items. Allemande, one of the sauces later classed among the French grandes sauces before Escoffier demoted it to a derivative of veloute, was a kitchen staple precisely because it could carry any mild garnish. Sweetbreads and cultivated mushrooms were both luxury goods in the 1890s American market, the first because offal that delicate spoils fast and demanded a trusted butcher, the second because commercial mushroom growing in the United States was still young and largely confined to cellars and caves near the big cities. Filling an omelet with either was a way of putting an expensive ingredient in front of a diner in a small, quickly served portion.