Step 1Heat the oven to 350 degrees. Divide the rabbit into front and hind legs and saddle, or have the butcher do this. Reserve the trimmings and rib cage. Place the bones and trimmings on a baking sheet or in a roasting pan. Drizzle with 2 teaspoons of olive oil. Roast until golden brown, about 30 minutes. Set aside.
Step 2Heat a heavy-bottomed Dutch-oven or similar ovenproof pan over medium-high heat. Season the rabbit legs and saddle with 1 teaspoon salt and one-fourth teaspoon pepper. Add 4 tablespoons of olive oil to the pan and sear the rabbit on all sides until golden brown, about 6 to 7 minutes. Remove and set aside.
Step 3Keep the pot over the heat and add the chopped carrot, celery, onion, 4 cloves of garlic and tomatoes. Cook until the ingredients have caramelized, about 10 to 12 minutes.
Step 4Deglaze with the red wine. Add the bay leaves, thyme, peppercorns, rabbit, roasted bones and trimmings and stock and bring to a simmer.
Step 5Cover and place the pot in the oven. Cook until the rabbit meat begins to pull away from the bones, about 2 to 2 1/2 hours. Remove the pot from the oven, remove the lid and let stand until the rabbit is cool enough to handle. Remove the saddle and legs from the pot and pull the meat from the bones. Set aside.
Step 6Strain the cooking liquid and skim any fat off the top.
Step 7While the rabbit braises, wrap the remaining 12 peeled garlic cloves in foil and roast in the oven for 45 minutes.
Step 8In a small pan, saute the pancetta until slightly crisp, about 12 to 15 minutes. Set aside. Blanch the carrots 2 minutes, peas 1 minute, fava beans 2 to 3 minutes. Set aside.
Step 9Bring a large pot of salted water to a rolling boil. Cook the pappardelle until just tender. Toss lightly with the remaining 2 tablespoons of olive oil and set aside.
Step 10In a skillet large enough to hold all the ingredients, add the braising liquid, mustard, pancetta and rabbit. Bring to a simmer and cook until the mixture begins to thicken slightly.
Step 11Add the carrots, peas, fava beans, roasted garlic and pasta. Cook until heated through. Finish with butter. Season with salt and pepper to taste. Divide into 8 bowls and serve at once.