Spread the ground meats around on a plate. Then wrap in film and place in the freezer for 1 hour.
In the meantime, melt the ½ tablespoon of butter in a small saucepan over medium heat, and when it begins to foam, add the fresh mushrooms and lower the heat, sautéing slowly for about 4 minutes. Squeeze all the water out of the now-plumped (formerly dried) mushrooms. Chop them finely and add them to the sauté pan. Continue to sauté for 1 more minute, then set aside.
Now make the milk infusion: in a medium saucepan, melt the butter over medium heat, and when it begins to foam, lower the heat to medium-low and add the onion, leek, and carrot. Cook the vegetables, stirring often, for about 4-5 minutes. They should look softened, but not colored.
Add the milk, cream, clove, bay leaf, and thyme, and bring to the boil over medium-high heat (watch this closely – milk can quickly boil over). Lower the heat to minimum, cover partially, and let simmer very gently for 20-30 minutes.
Cut the bread slices into cubes about the size of your thumbnail. Preheat oven to 320°F (160°C). Strain the milk into a medium bowl and set aside.
Remove the meats from the freezer and place them in the bowl of a food processor. Process them by alternating the pulse command and running continuously for about 3-4 minutes.
Add the egg and egg whites, and continue to process. Then run the machine continuously, and very slowly and carefully pour in the still-hot milk. Continue to process, stop to add the bread cubes, and then continue until you have a smooth mixture.
Add the salt, pepper, nutmeg, and Port, and pulse once more to mix.
Stir in the mushrooms, and turn the mixture into a buttered terrine mold or loaf pan.
Cover with aluminum foil or the terrine mold’s cover and bake in a hot water bath for 40 minutes, or until a knife poked into the center comes out clean.
When ready to eat, slice the boudin into 1-inch slices (2.5cm), and sauté both sides of the boudin in a bit of butter over medium heat. Or roll cubes of boudin into puff pastry and bake according to pastry instructions. Bon app’!