In a bowl, combine the livers and milk to cover. Soak, refrigerated, for at least 8 hours or overnight. Drain well.
In a large, heavy skillet, heat the oil over medium-high heat. Add the livers, onions, thyme, bay leaves, 1 teaspoon of the salt, and the white pepper. Cook, stirring occasionally, until the livers are just slightly pink and the onions are soft, about 5 minutes. Carefully add the brandy and cook until the liquid is evaporated and the livers are cooked through but still tender. Remove from the heat and spread on a large plate tocool. Transfer to a food processor. Add the cream cheese and butter, and process on high speed until smooth. Add the remaining 1/2 teaspoon of salt and pulse to blend.
Line a 5-cup loaf pan (8 1/2 by 4 1/2 by 3-inches) with plastic wrap, leaving an overhang of several inches. Spoon the mixture into the terrine, smoothing out with a rubber spatula. Wrap the terrine with the overhanging wrap and chill until firm, at least 8 hours.
To serve, unwrap the terrine, lift from the mold, and slice with a sharp knife. Place 1 slice on each plate with a heaping tablespoon of the mustard sauce and serve with the croutons or crackers.
Yield: 5 cups, 10 to 12 servings