In a small heavy saucepan over low heat, melt 2 tablespoons/1 ounce/28 grams of the butter. Add the onion and cook, covered, for 5 minutes until translucent. Stir in the cream and garlic and simmer, covered, for another 5 minutes.

Add the remaining butter and heat, stirring, just until it is melted and the mixture is well combined. Scrape the mixture into a medium bowl. Add the livers, salt, and pepper and, with an immersion blender, puree the mixture until smooth. Alternatively, use a blender or food processor, scraping down the sides of the bowl as needed.

Set a fine-mesh strainer over a medium bowl and press the liver mixture through with the back of a spoon. Scrape the mixture into the terrines. Cover them tightly with aluminum foil and set them in the baking pan. Fill the pan with boiling water to come halfway up the sides of the terrines.