Prepare all ingredients.
In a medium saucepan, combine the chicken livers and sliced onion. Add about 3 cups water and bring to a boil.
Reduce heat to low, cover, and simmer until the liver is tender, approximately 20 minutes. Check with an instant-read thermometer; it should read at least 74°C (165°F).
Remove from heat and drain. Discard the sliced onion and remove any hard portions of the livers.
Transfer cooked livers to a food processor and blend until smooth.
Add softened butter, chopped onion, sherry, salt, pepper, and mace. Pulse until well combined.
Shape the mixture into a mound using lightly greased hands. Place it onto a serving platter, then cover and refrigerate for about 1 hour before serving.
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