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Pending the author's reply, chopped liver is usually spread on pieces or slices of bread (in this case challah), with optional garnishes mentioned on the same line in her menu (fried onions, gribenes).
That said, you could slice the challah (whole loaf) in half horizontally, spread a layer of chopped liver spread, top with remaining bread, and cut the loaf into canapes or hor d'oeuvres (maybe with toothpicks if the challah tops fall off). Let people still choose whether to have either of the garnishes.