The duxelles: Cook the chopped onion gently in the butter without permitting it to color for 20 minutes or so, add the mushrooms, turning the flame high and tossing and stirring until the mushrooms’ moisture has been evaporated, season, add parsley, and cook for a couple of minutes still over low flame. Stir in a few drops of lemon juice.
Stew the shredded sorrel in butter, stirring occasionally, for 10 minutes or so—longer, if necessary, until all excess moisture has been evaporated and the sorrel is reduced to a purée.
Mix the squeezed breadcrumb paste thoroughly with the garlic, which has first been pounded to a purée in a mortar.
Combine all the above with the remaining ingredients in a large mixing bowl, mix thoroughly with your hands, turn out into the buttered terrine, tapping the bottom of it several times against a table top, the blows muffled by the intervention of several layers of towel, to settle the contents, and poach for about 1½ hours in a medium oven (350° to 375°), the terrine put into a bain-marie. Cool under pressure as for the rabbit terrine and refrigerate for a day before serving.