Preheat the oven to 300ºF.

In a large sauté pan heat the olive oil over medium-high heat. Add the onions and cook, stirring, until soft, 3 minutes. Add the garlic and cook for 30 seconds. Add the mushrooms, 1/2 teaspoon of the salt, and 1/4 teaspoon of the pepper, and cook until they give off their liquid, about 5 minutes. Remove the mushrooms from the pan and cool.

In a mixing bowl combine the beef, pork, buttermilk, sage, remaining 1/2 teaspoon of salt, and 1/4 teaspoon of pepper, egg whites and the cooled mushrooms. Stir well to blend. Line a 6-cup (12 x 3 x 3-inch) terrine mold with prosciutto slices, overhanging the strips over the edges of the mold. Pour the paté filling into the mold and cover with the prosciutto. Wrap the terrine loosely in plastic wrap. Top with an equal sized terrine, or a piece of cardboard wrapped in aluminum foil. Place a brick on top of the terrine mold and bake for 1 hour. Remove from the oven and allow paté to stand for 30 minutes before removing the weight and unwrapping. Pour off any accumulated juices and allow paté to cool completely. Cover paté with plastic wrap and refrigerate until completely cooled. Serve slices of paté either cold or at room temperature, lightly drizzled with the Cognac Mustard Sauce, with cornichons on the side.

COGNAC MUSTARD SAUCE: In a bowl, combine all the ingredients and whisk until well incorporated.

Yield: about 1 cup of sauce