In a soup pot, heat the olive oil and butter together until the butter foams and subsides. Add the onions, porcini stems, and parsley, reduce the heat to medium and cook, stirring regularly, until the mushrooms are very soft, occasionally moistening the pan with small amounts of the chicken stock.
Once the vegetables are very soft, remove from heat and place the mixture in a blender. Blend to a fine consistency and return to the soup pot over low heat. Stir in the remaining chicken broth and season with salt and pepper. Bring to a boil, add the reserved porcini caps and simmer 20 minutes.
In a small mixing bowl mix the mascarpone with the rosemary and the pepper. Smear the mascarpone mixture on to the baguette slices and place 3 smeared slices in each of 4 bowls. Ladle the soup over the crostini serve immediately.
In a large, heavy-bottomed saucepan, heat the oil over high heat until smoking. Add all the chicken parts and brown all over, stirring to avoid burning. Remove the chicken and reserve. Add the carrots, onions, and celery to the pot and cook until soft and browned. Return the chicken to the pot and add 3 quarts of water, the tomato paste, peppercorns, and parsley. Stir with a wooden spoon to dislodge the browned chicken and vegetables bits from the bottom of the pan. Bring almost to a boil, then reduce heat and cook at a low simmer until reduced by half, about 2 hours, occasionally skimming excess fat. Remove from heat, strain, and press on the solids with the bottom of a ladle to extract out all liquids. Stir the stock to facilitate cooling and set aside. Refrigerate stock in small containers for up to a week or freeze for up to a month.
Recipe copyright 2000, Mario Batali. All Rights Reserved.