• Heat oven to 350°. Cook onions slowly in butter until translucent. Add clove crowns and thyme and cook for a few more minutes. Let cool. Combine with remaining ingredients. Cook a small piece to check seasoning.
• Line a terrine mold (11½- by 3¼- by 3 inches) or small loaf pan (5 cups volume) with thinly sliced prosciutto, slightly overlapping the pieces and letting slices hang about 2 inches over edge of mold.
• Fill lined mold with terrine filling.
• Press filling down and fold lining neatly over the filling. Cover with terrine lid or foil and place in a deep baking dish. Fill the baking dish with boiling water so that water comes halfway up the sides of the terrine pan. Bake until internal temperature reaches 145 to 155° (45 to 75 minutes). Let cool a few minutes.
• Press with a terrine press or wrap a piece of cardboard that is cut the size of the terrine opening with plastic wrap and lay on top of the terrine, weight down with a brick or other heavy object and refrigerate overnight. Run a hot knife around the edges of the terrine to unmold.
• Serves 20 or more. Serve in slices with pickled vegetables, mostarda and crackers or bread.